Implementation of for...of loops (#704)

* Initial implementation of for...of loop

* Initial implementation of for...of loop

* Extend for...of to support var, let, and const lhs

* Use cached well known symbols

* Nest use statements

* Nest use statements

* Add tests.

* Initial implementation of for...of loop

* Extend for...of to support var, let, and const lhs

* Use cached well known symbols

* Nest use statements

* Nest use statements

* Add tests.

* Add tests, cache iterator prototypes, pull common iterator functions out into their own file to allow for repeated use.

* Initial implementation of for...of loop

* Extend for...of to support var, let, and const lhs

* Use cached well known symbols

* Nest use statements

* Nest use statements

* Add tests.

* Add tests, cache iterator prototypes, pull common iterator functions out into their own file to allow for repeated use.

* Initial implementation of for...of loop

* Extend for...of to support var, let, and const lhs

* Use cached well known symbols

* Initial implementation of for...of loop

* Nest use statements

* Add string iterator

* Clean up merge

* Use ctx.global_iterator()

* Merge upstream

* Use u32 as array next index

* Use into

* Use boa::Result

* Tidy up use statement

* Tidy up use statement

boa/src/builtins/array/array_iterator.rs

@@ -0,0 +1,139 @@
+use crate::{
+    builtins::{function::make_builtin_fn, iterable::create_iter_result_object, Array, Value},
+    object::ObjectData,
+    property::{Attribute, Property},
+    BoaProfiler, Context, Result,
+};
+use gc::{Finalize, Trace};
+
+#[derive(Debug, Clone, Finalize, Trace)]
+pub enum ArrayIterationKind {
+    Key,
+    Value,
+    KeyAndValue,
+}
+
+/// The Array Iterator object represents an iteration over an array. It implements the iterator protocol.
+///
+/// More information:
+///  - [ECMAScript reference][spec]
+///
+/// [spec]: https://tc39.es/ecma262/#sec-array-iterator-objects
+#[derive(Debug, Clone, Finalize, Trace)]
+pub struct ArrayIterator {
+    array: Value,
+    next_index: u32,
+    kind: ArrayIterationKind,
+}
+
+impl ArrayIterator {
+    pub(crate) const NAME: &'static str = "ArrayIterator";
+
+    fn new(array: Value, kind: ArrayIterationKind) -> Self {
+        ArrayIterator {
+            array,
+            kind,
+            next_index: 0,
+        }
+    }
+
+    /// CreateArrayIterator( array, kind )
+    ///
+    /// Creates a new iterator over the given array.
+    ///
+    /// More information:
+    ///  - [ECMA reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-createarrayiterator
+    pub(crate) fn create_array_iterator(
+        ctx: &Context,
+        array: Value,
+        kind: ArrayIterationKind,
+    ) -> Result<Value> {
+        let array_iterator = Value::new_object(Some(ctx.global_object()));
+        array_iterator.set_data(ObjectData::ArrayIterator(Self::new(array, kind)));
+        array_iterator
+            .as_object_mut()
+            .expect("array iterator object")
+            .set_prototype_instance(ctx.iterator_prototypes().array_iterator().into());
+        Ok(array_iterator)
+    }
+
+    /// %ArrayIteratorPrototype%.next( )
+    ///
+    /// Gets the next result in the array.
+    ///
+    /// More information:
+    ///  - [ECMA reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-%arrayiteratorprototype%.next
+    pub(crate) fn next(this: &Value, _args: &[Value], ctx: &mut Context) -> Result<Value> {
+        if let Value::Object(ref object) = this {
+            let mut object = object.borrow_mut();
+            if let Some(array_iterator) = object.as_array_iterator_mut() {
+                let index = array_iterator.next_index;
+                if array_iterator.array.is_undefined() {
+                    return Ok(create_iter_result_object(ctx, Value::undefined(), true));
+                }
+                let len = array_iterator
+                    .array
+                    .get_field("length")
+                    .as_number()
+                    .ok_or_else(|| ctx.construct_type_error("Not an array"))?
+                    as u32;
+                if array_iterator.next_index >= len {
+                    array_iterator.array = Value::undefined();
+                    return Ok(create_iter_result_object(ctx, Value::undefined(), true));
+                }
+                array_iterator.next_index = index + 1;
+                match array_iterator.kind {
+                    ArrayIterationKind::Key => {
+                        Ok(create_iter_result_object(ctx, index.into(), false))
+                    }
+                    ArrayIterationKind::Value => {
+                        let element_value = array_iterator.array.get_field(index);
+                        Ok(create_iter_result_object(ctx, element_value, false))
+                    }
+                    ArrayIterationKind::KeyAndValue => {
+                        let element_value = array_iterator.array.get_field(index);
+                        let result = Array::make_array(
+                            &Value::new_object(Some(ctx.global_object())),
+                            &[index.into(), element_value],
+                            ctx,
+                        )?;
+                        Ok(create_iter_result_object(ctx, result, false))
+                    }
+                }
+            } else {
+                ctx.throw_type_error("`this` is not an ArrayIterator")
+            }
+        } else {
+            ctx.throw_type_error("`this` is not an ArrayIterator")
+        }
+    }
+
+    /// Create the %ArrayIteratorPrototype% object
+    ///
+    /// More information:
+    ///  - [ECMA reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-%arrayiteratorprototype%-object
+    pub(crate) fn create_prototype(ctx: &mut Context, iterator_prototype: Value) -> Value {
+        let global = ctx.global_object();
+        let _timer = BoaProfiler::global().start_event(Self::NAME, "init");
+
+        // Create prototype
+        let array_iterator = Value::new_object(Some(global));
+        make_builtin_fn(Self::next, "next", &array_iterator, 0, ctx);
+        array_iterator
+            .as_object_mut()
+            .expect("array iterator prototype object")
+            .set_prototype_instance(iterator_prototype);
+
+        let to_string_tag = ctx.well_known_symbols().to_string_tag_symbol();
+        let to_string_tag_property =
+            Property::data_descriptor(Value::string("Array Iterator"), Attribute::CONFIGURABLE);
+        array_iterator.set_property(to_string_tag, to_string_tag_property);
+        array_iterator
+    }
+}

boa/src/builtins/array/mod.rs

@@ -9,11 +9,13 @@
 //! [spec]: https://tc39.es/ecma262/#sec-array-objects
 //! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array
 
+pub mod array_iterator;
 #[cfg(test)]
 mod tests;
 
 use super::function::{make_builtin_fn, make_constructor_fn};
 use crate::{
+    builtins::array::array_iterator::{ArrayIterationKind, ArrayIterator},
     object::{ObjectData, PROTOTYPE},
     property::{Attribute, Property},
     value::{same_value_zero, Value},
@@ -1091,6 +1093,60 @@ impl Array {
         Ok(accumulator)
     }
 
+    /// `Array.prototype.values( )`
+    ///
+    /// The values method returns an iterable that iterates over the values in the array.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-array.prototype.values
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/values
+    pub(crate) fn values(
+        this: &Value,
+        _args: &[Value],
+        interpreter: &mut Context,
+    ) -> Result<Value> {
+        ArrayIterator::create_array_iterator(interpreter, this.clone(), ArrayIterationKind::Value)
+    }
+
+    /// `Array.prototype.keys( )`
+    ///
+    /// The keys method returns an iterable that iterates over the indexes in the array.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-array.prototype.values
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/values
+    pub(crate) fn keys(this: &Value, _args: &[Value], interpreter: &mut Context) -> Result<Value> {
+        ArrayIterator::create_array_iterator(interpreter, this.clone(), ArrayIterationKind::Key)
+    }
+
+    /// `Array.prototype.entries( )`
+    ///
+    /// The entries method returns an iterable that iterates over the key-value pairs in the array.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-array.prototype.values
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/values
+    pub(crate) fn entries(
+        this: &Value,
+        _args: &[Value],
+        interpreter: &mut Context,
+    ) -> Result<Value> {
+        ArrayIterator::create_array_iterator(
+            interpreter,
+            this.clone(),
+            ArrayIterationKind::KeyAndValue,
+        )
+    }
+
     /// Initialise the `Array` object on the global object.
     #[inline]
     pub(crate) fn init(interpreter: &mut Context) -> (&'static str, Value) {
@@ -1137,6 +1193,15 @@ impl Array {
             2,
             interpreter,
         );
+        make_builtin_fn(Self::values, "values", &prototype, 0, interpreter);
+        make_builtin_fn(Self::keys, "keys", &prototype, 0, interpreter);
+        make_builtin_fn(Self::entries, "entries", &prototype, 0, interpreter);
+
+        let symbol_iterator = interpreter.well_known_symbols().iterator_symbol();
+        prototype.set_property(
+            symbol_iterator,
+            Property::default().value(prototype.get_field("values")),
+        );
 
         let array = make_constructor_fn(
             Self::NAME,

boa/src/builtins/array/tests.rs

@@ -1078,3 +1078,135 @@ fn call_array_constructor_with_one_argument() {
     // let result = forward(&mut engine, "one.length");
     // assert_eq!(result, "1");
 }
+
+#[test]
+fn array_values_simple() {
+    let mut engine = Context::new();
+    let init = r#"
+        var iterator = [1, 2, 3].values();
+        var next = iterator.next();
+    "#;
+    forward(&mut engine, init);
+    assert_eq!(forward(&mut engine, "next.value"), "1");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "2");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "3");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "true");
+}
+
+#[test]
+fn array_keys_simple() {
+    let mut engine = Context::new();
+    let init = r#"
+        var iterator = [1, 2, 3].keys();
+        var next = iterator.next();
+    "#;
+    forward(&mut engine, init);
+    assert_eq!(forward(&mut engine, "next.value"), "0");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "1");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "2");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "true");
+}
+
+#[test]
+fn array_entries_simple() {
+    let mut engine = Context::new();
+    let init = r#"
+        var iterator = [1, 2, 3].entries();
+        var next = iterator.next();
+    "#;
+    forward(&mut engine, init);
+    assert_eq!(forward(&mut engine, "next.value"), "[ 0, 1 ]");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "[ 1, 2 ]");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "[ 2, 3 ]");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "true");
+}
+
+#[test]
+fn array_values_empty() {
+    let mut engine = Context::new();
+    let init = r#"
+        var iterator = [].values();
+        var next = iterator.next();
+    "#;
+    forward(&mut engine, init);
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "true");
+}
+
+#[test]
+fn array_values_sparse() {
+    let mut engine = Context::new();
+    let init = r#"
+        var array = Array();
+        array[3] = 5;
+        var iterator = array.values();
+        var next = iterator.next();
+    "#;
+    forward(&mut engine, init);
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "5");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "true");
+}
+
+#[test]
+fn array_symbol_iterator() {
+    let mut engine = Context::new();
+    let init = r#"
+        var iterator = [1, 2, 3][Symbol.iterator]();
+        var next = iterator.next();
+    "#;
+    forward(&mut engine, init);
+    assert_eq!(forward(&mut engine, "next.value"), "1");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "2");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "3");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iterator.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "true");
+}
+
+#[test]
+fn array_values_symbol_iterator() {
+    let mut engine = Context::new();
+    let init = r#"
+        var iterator = [1, 2, 3].values();
+        iterator === iterator[Symbol.iterator]();
+    "#;
+    assert_eq!(forward(&mut engine, init), "true");
+}

boa/src/builtins/iterable/mod.rs

@@ -0,0 +1,154 @@
+use crate::{
+    builtins::string::string_iterator::StringIterator,
+    builtins::{
+        function::{BuiltInFunction, Function, FunctionFlags},
+        ArrayIterator,
+    },
+    object::GcObject,
+    object::{Object, PROTOTYPE},
+    property::Property,
+    BoaProfiler, Context, Result, Value,
+};
+
+#[derive(Debug, Default)]
+pub struct IteratorPrototypes {
+    iterator_prototype: GcObject,
+    array_iterator: GcObject,
+    string_iterator: GcObject,
+}
+
+impl IteratorPrototypes {
+    pub fn init(ctx: &mut Context) -> Self {
+        let iterator_prototype = create_iterator_prototype(ctx);
+        Self {
+            iterator_prototype: iterator_prototype
+                .as_gc_object()
+                .expect("Iterator prototype is not an object"),
+            array_iterator: ArrayIterator::create_prototype(ctx, iterator_prototype.clone())
+                .as_gc_object()
+                .expect("Array Iterator Prototype is not an object"),
+            string_iterator: StringIterator::create_prototype(ctx, iterator_prototype)
+                .as_gc_object()
+                .expect("String Iterator Prototype is not an object"),
+        }
+    }
+
+    pub fn array_iterator(&self) -> GcObject {
+        self.array_iterator.clone()
+    }
+
+    pub fn iterator_prototype(&self) -> GcObject {
+        self.iterator_prototype.clone()
+    }
+
+    pub fn string_iterator(&self) -> GcObject {
+        self.string_iterator.clone()
+    }
+}
+
+/// CreateIterResultObject( value, done )
+///
+/// Generates an object supporting the IteratorResult interface.
+pub fn create_iter_result_object(ctx: &mut Context, value: Value, done: bool) -> Value {
+    let object = Value::new_object(Some(ctx.global_object()));
+    let value_property = Property::default().value(value);
+    let done_property = Property::default().value(Value::boolean(done));
+    object.set_property("value", value_property);
+    object.set_property("done", done_property);
+    object
+}
+
+/// Get an iterator record
+pub fn get_iterator(ctx: &mut Context, iterable: Value) -> Result<IteratorRecord> {
+    let iterator_function = iterable
+        .get_property(ctx.well_known_symbols().iterator_symbol())
+        .and_then(|mut p| p.value.take())
+        .ok_or_else(|| ctx.construct_type_error("Not an iterable"))?;
+    let iterator_object = ctx.call(&iterator_function, &iterable, &[])?;
+    let next_function = iterator_object
+        .get_property("next")
+        .and_then(|mut p| p.value.take())
+        .ok_or_else(|| ctx.construct_type_error("Could not find property `next`"))?;
+    Ok(IteratorRecord::new(iterator_object, next_function))
+}
+
+/// Create the %IteratorPrototype% object
+///
+/// More information:
+///  - [ECMA reference][spec]
+///
+/// [spec]: https://tc39.es/ecma262/#sec-%iteratorprototype%-object
+fn create_iterator_prototype(ctx: &mut Context) -> Value {
+    let global = ctx.global_object();
+    let _timer = BoaProfiler::global().start_event("Iterator Prototype", "init");
+
+    let iterator_prototype = Value::new_object(Some(global));
+    let mut function = Object::function(
+        Function::BuiltIn(
+            BuiltInFunction(|v, _, _| Ok(v.clone())),
+            FunctionFlags::CALLABLE,
+        ),
+        global.get_field("Function").get_field(PROTOTYPE),
+    );
+    function.insert_field("length", Value::from(0));
+    function.insert_field("name", Value::string("[Symbol.iterator]"));
+
+    let symbol_iterator = ctx.well_known_symbols().iterator_symbol();
+    iterator_prototype.set_field(symbol_iterator, Value::from(function));
+    iterator_prototype
+}
+
+#[derive(Debug)]
+pub struct IteratorRecord {
+    iterator_object: Value,
+    next_function: Value,
+}
+
+impl IteratorRecord {
+    fn new(iterator_object: Value, next_function: Value) -> Self {
+        Self {
+            iterator_object,
+            next_function,
+        }
+    }
+
+    /// Get the next value in the iterator
+    ///
+    /// More information:
+    ///  - [ECMA reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-iteratornext
+    pub(crate) fn next(&self, ctx: &mut Context) -> Result<IteratorResult> {
+        let next = ctx.call(&self.next_function, &self.iterator_object, &[])?;
+        let done = next
+            .get_property("done")
+            .and_then(|mut p| p.value.take())
+            .and_then(|v| v.as_boolean())
+            .ok_or_else(|| ctx.construct_type_error("Could not find property `done`"))?;
+        let next_result = next
+            .get_property("value")
+            .and_then(|mut p| p.value.take())
+            .unwrap_or_default();
+        Ok(IteratorResult::new(next_result, done))
+    }
+}
+
+#[derive(Debug)]
+pub struct IteratorResult {
+    value: Value,
+    done: bool,
+}
+
+impl IteratorResult {
+    fn new(value: Value, done: bool) -> Self {
+        Self { value, done }
+    }
+
+    pub fn is_done(&self) -> bool {
+        self.done
+    }
+
+    pub fn value(self) -> Value {
+        self.value
+    }
+}

boa/src/builtins/mod.rs

@@ -9,6 +9,7 @@ pub mod error;
 pub mod function;
 pub mod global_this;
 pub mod infinity;
+pub mod iterable;
 pub mod json;
 pub mod map;
 pub mod math;
@@ -21,7 +22,7 @@ pub mod symbol;
 pub mod undefined;
 
 pub(crate) use self::{
-    array::Array,
+    array::{array_iterator::ArrayIterator, Array},
     bigint::BigInt,
     boolean::Boolean,
     console::Console,
@@ -49,6 +50,7 @@ pub fn init(interpreter: &mut Context) {
         // The `Function` global must be initialized before other types.
         function::init,
         Object::init,
+        Symbol::init,
         Array::init,
         BigInt::init,
         Boolean::init,
@@ -59,7 +61,6 @@ pub fn init(interpreter: &mut Context) {
         Number::init,
         RegExp::init,
         String::init,
-        Symbol::init,
         Console::init,
         // Global error types.
         Error::init,

boa/src/builtins/string/mod.rs

@@ -9,10 +9,14 @@
 //! [spec]: https://tc39.es/ecma262/#sec-string-object
 //! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String
 
+pub mod string_iterator;
 #[cfg(test)]
 mod tests;
 
 use super::function::{make_builtin_fn, make_constructor_fn};
+use crate::builtins::function::{BuiltInFunction, Function, FunctionFlags};
+use crate::builtins::string::string_iterator::StringIterator;
+use crate::object::PROTOTYPE;
 use crate::{
     builtins::RegExp,
     object::{Object, ObjectData},
@@ -21,12 +25,42 @@ use crate::{
     BoaProfiler, Context, Result,
 };
 use regex::Regex;
-use std::string::String as StdString;
 use std::{
+    char::decode_utf16,
     cmp::{max, min},
     f64::NAN,
+    string::String as StdString,
 };
 
+pub(crate) fn code_point_at(string: RcString, position: i32) -> Option<(u32, u8, bool)> {
+    let size = string.encode_utf16().count() as i32;
+    if position < 0 || position >= size {
+        return None;
+    }
+    let mut encoded = string.encode_utf16();
+    let first = encoded.nth(position as usize)?;
+    if !is_leading_surrogate(first) && !is_trailing_surrogate(first) {
+        return Some((first as u32, 1, false));
+    }
+    if is_trailing_surrogate(first) || position + 1 == size {
+        return Some((first as u32, 1, true));
+    }
+    let second = encoded.next()?;
+    if !is_trailing_surrogate(second) {
+        return Some((first as u32, 1, true));
+    }
+    let cp = (first as u32 - 0xD800) * 0x400 + (second as u32 - 0xDC00) + 0x10000;
+    Some((cp, 2, false))
+}
+
+fn is_leading_surrogate(value: u16) -> bool {
+    value >= 0xD800 && value <= 0xDBFF
+}
+
+fn is_trailing_surrogate(value: u16) -> bool {
+    value >= 0xDC00 && value <= 0xDFFF
+}
+
 /// JavaScript `String` implementation.
 #[derive(Debug, Clone, Copy)]
 pub(crate) struct String;
@@ -72,7 +106,7 @@ impl String {
             None => RcString::default(),
         };
 
-        let length = string.chars().count();
+        let length = string.encode_utf16().count();
 
         this.set_field("length", Value::from(length as i32));
 
@@ -922,7 +956,7 @@ impl String {
                 .expect("failed to get argument for String method")
                 .to_integer(ctx)? as i32
         };
-        let length = primitive_val.chars().count() as i32;
+        let length = primitive_val.encode_utf16().count() as i32;
         // If less than 2 args specified, end is the length of the this object converted to a String
         let end = if args.len() < 2 {
             length
@@ -940,12 +974,14 @@ impl String {
         let to = max(final_start, final_end) as usize;
         // Extract the part of the string contained between the start index and the end index
         // where start is guaranteed to be smaller or equals to end
-        let extracted_string: StdString = primitive_val
-            .chars()
-            .skip(from)
-            .take(to.wrapping_sub(from))
-            .collect();
-        Ok(Value::from(extracted_string))
+        let extracted_string: std::result::Result<StdString, _> = decode_utf16(
+            primitive_val
+                .encode_utf16()
+                .skip(from)
+                .take(to.wrapping_sub(from)),
+        )
+        .collect();
+        Ok(Value::from(extracted_string.expect("Invalid string")))
     }
 
     /// `String.prototype.substr( start[, length] )`
@@ -1062,6 +1098,10 @@ impl String {
         RegExp::match_all(&re, this.to_string(ctx)?.to_string())
     }
 
+    pub(crate) fn iterator(this: &Value, _args: &[Value], ctx: &mut Context) -> Result<Value> {
+        StringIterator::create_string_iterator(ctx, this.clone())
+    }
+
     /// Initialise the `String` object on the global object.
     #[inline]
     pub(crate) fn init(interpreter: &mut Context) -> (&'static str, Value) {
@@ -1118,6 +1158,15 @@ impl String {
         make_builtin_fn(Self::match_all, "matchAll", &prototype, 1, interpreter);
         make_builtin_fn(Self::replace, "replace", &prototype, 2, interpreter);
 
+        let symbol_iterator = interpreter.well_known_symbols().iterator_symbol();
+        let mut function = Object::function(
+            Function::BuiltIn(BuiltInFunction(Self::iterator), FunctionFlags::CALLABLE),
+            global.get_field("Function").get_field(PROTOTYPE),
+        );
+        function.insert_field("length", Value::from(0));
+        function.insert_field("name", Value::string("[Symbol.iterator]"));
+        prototype.set_field(symbol_iterator, Value::from(function));
+
         let string_object = make_constructor_fn(
             Self::NAME,
             Self::LENGTH,

boa/src/builtins/string/string_iterator.rs

@@ -0,0 +1,89 @@
+use crate::builtins::string::code_point_at;
+use crate::{
+    builtins::{function::make_builtin_fn, iterable::create_iter_result_object},
+    object::ObjectData,
+    property::{Attribute, Property},
+    BoaProfiler, Context, Result, Value,
+};
+use gc::{Finalize, Trace};
+
+#[derive(Debug, Clone, Finalize, Trace)]
+pub struct StringIterator {
+    string: Value,
+    next_index: i32,
+}
+
+impl StringIterator {
+    fn new(string: Value) -> Self {
+        Self {
+            string,
+            next_index: 0,
+        }
+    }
+
+    pub fn create_string_iterator(ctx: &mut Context, string: Value) -> Result<Value> {
+        let string_iterator = Value::new_object(Some(ctx.global_object()));
+        string_iterator.set_data(ObjectData::StringIterator(Self::new(string)));
+        string_iterator
+            .as_object_mut()
+            .expect("array iterator object")
+            .set_prototype_instance(ctx.iterator_prototypes().string_iterator().into());
+        Ok(string_iterator)
+    }
+
+    pub fn next(this: &Value, _args: &[Value], ctx: &mut Context) -> Result<Value> {
+        if let Value::Object(ref object) = this {
+            let mut object = object.borrow_mut();
+            if let Some(string_iterator) = object.as_string_iterator_mut() {
+                if string_iterator.string.is_undefined() {
+                    return Ok(create_iter_result_object(ctx, Value::undefined(), true));
+                }
+                let native_string = string_iterator.string.to_string(ctx)?;
+                let len = native_string.encode_utf16().count() as i32;
+                let position = string_iterator.next_index;
+                if position >= len {
+                    string_iterator.string = Value::undefined();
+                    return Ok(create_iter_result_object(ctx, Value::undefined(), true));
+                }
+                let (_, code_unit_count, _) =
+                    code_point_at(native_string, position).expect("Invalid code point position");
+                string_iterator.next_index += code_unit_count as i32;
+                let result_string = crate::builtins::string::String::substring(
+                    &string_iterator.string,
+                    &[position.into(), string_iterator.next_index.into()],
+                    ctx,
+                )?;
+                Ok(create_iter_result_object(ctx, result_string, false))
+            } else {
+                ctx.throw_type_error("`this` is not an ArrayIterator")
+            }
+        } else {
+            ctx.throw_type_error("`this` is not an ArrayIterator")
+        }
+    }
+
+    /// Create the %ArrayIteratorPrototype% object
+    ///
+    /// More information:
+    ///  - [ECMA reference][spec]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-%arrayiteratorprototype%-object
+    pub(crate) fn create_prototype(ctx: &mut Context, iterator_prototype: Value) -> Value {
+        let global = ctx.global_object();
+        let _timer = BoaProfiler::global().start_event("String Iterator", "init");
+
+        // Create prototype
+        let array_iterator = Value::new_object(Some(global));
+        make_builtin_fn(Self::next, "next", &array_iterator, 0, ctx);
+        array_iterator
+            .as_object_mut()
+            .expect("array iterator prototype object")
+            .set_prototype_instance(iterator_prototype);
+
+        let to_string_tag = ctx.well_known_symbols().to_string_tag_symbol();
+        let to_string_tag_property =
+            Property::data_descriptor(Value::string("String Iterator"), Attribute::CONFIGURABLE);
+        array_iterator.set_property(to_string_tag, to_string_tag_property);
+        array_iterator
+    }
+}

boa/src/builtins/string/tests.rs

@@ -679,3 +679,102 @@ fn last_index_non_integer_position_argument() {
     );
     assert_eq!(forward(&mut engine, "'abcx'.lastIndexOf('x', null)"), "3");
 }
+
+#[test]
+fn empty_iter() {
+    let mut engine = Context::new();
+    let init = r#"
+        let iter = new String()[Symbol.iterator]();
+        let next = iter.next();
+    "#;
+    forward(&mut engine, init);
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "true");
+}
+
+#[test]
+fn ascii_iter() {
+    let mut engine = Context::new();
+    let init = r#"
+        let iter = new String("Hello World")[Symbol.iterator]();
+        let next = iter.next();
+    "#;
+    forward(&mut engine, init);
+    assert_eq!(forward(&mut engine, "next.value"), "\"H\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"e\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"l\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"l\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"o\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\" \"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"W\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"o\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"r\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"l\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"d\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "true");
+}
+
+#[test]
+fn unicode_iter() {
+    let mut engine = Context::new();
+    let init = r#"
+        let iter = new String("C🙂🙂l W🙂rld")[Symbol.iterator]();
+        let next = iter.next();
+    "#;
+    forward(&mut engine, init);
+    assert_eq!(forward(&mut engine, "next.value"), "\"C\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"🙂\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"🙂\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"l\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\" \"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"W\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"🙂\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"r\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"l\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "\"d\"");
+    assert_eq!(forward(&mut engine, "next.done"), "false");
+    forward(&mut engine, "next = iter.next()");
+    assert_eq!(forward(&mut engine, "next.value"), "undefined");
+    assert_eq!(forward(&mut engine, "next.done"), "true");
+}

boa/src/context.rs

@@ -4,6 +4,7 @@ use crate::{
     builtins::{
         self,
         function::{Function, FunctionFlags, NativeFunction},
+        iterable::IteratorPrototypes,
         symbol::{Symbol, WellKnownSymbols},
         Console,
     },
@@ -50,6 +51,8 @@ pub struct Context {
 
     /// Cached well known symbols
     well_known_symbols: WellKnownSymbols,
+
+    iterator_prototypes: IteratorPrototypes,
 }
 
 impl Default for Context {
@@ -63,13 +66,14 @@ impl Default for Context {
             symbol_count,
             console: Console::default(),
             well_known_symbols,
+            iterator_prototypes: IteratorPrototypes::default(),
         };
 
         // Add new builtIns to Context Realm
         // At a later date this can be removed from here and called explicitly,
         // but for now we almost always want these default builtins
         context.create_intrinsics();
-
+        context.iterator_prototypes = IteratorPrototypes::init(&mut context);
         context
     }
 }
@@ -517,4 +521,9 @@ impl Context {
     pub fn well_known_symbols(&self) -> &WellKnownSymbols {
         &self.well_known_symbols
     }
+
+    #[inline]
+    pub fn iterator_prototypes(&self) -> &IteratorPrototypes {
+        &self.iterator_prototypes
+    }
 }

boa/src/object/gcobject.rs

@@ -29,7 +29,7 @@ pub type Ref<'object> = GcCellRef<'object, Object>;
 pub type RefMut<'object> = GcCellRefMut<'object, Object>;
 
 /// Garbage collected `Object`.
-#[derive(Trace, Finalize, Clone)]
+#[derive(Trace, Finalize, Clone, Default)]
 pub struct GcObject(Gc<GcCell<Object>>);
 
 // This is needed for the call method since we cannot mutate the function itself since we

boa/src/object/mod.rs

@@ -1,7 +1,10 @@
 //! This module implements the Rust representation of a JavaScript object.
 
 use crate::{
-    builtins::{function::Function, map::ordered_map::OrderedMap, BigInt, Date, RegExp},
+    builtins::{
+        array::array_iterator::ArrayIterator, function::Function, map::ordered_map::OrderedMap,
+        string::string_iterator::StringIterator, BigInt, Date, RegExp,
+    },
     property::{Property, PropertyKey},
     value::{RcBigInt, RcString, RcSymbol, Value},
     BoaProfiler,
@@ -62,12 +65,14 @@ pub struct Object {
 #[derive(Debug, Trace, Finalize)]
 pub enum ObjectData {
     Array,
+    ArrayIterator(ArrayIterator),
     Map(OrderedMap<Value, Value>),
     RegExp(Box<RegExp>),
     BigInt(RcBigInt),
     Boolean(bool),
     Function(Function),
     String(RcString),
+    StringIterator(StringIterator),
     Number(f64),
     Symbol(RcSymbol),
     Error,
@@ -84,10 +89,12 @@ impl Display for ObjectData {
             "{}",
             match self {
                 Self::Array => "Array",
+                Self::ArrayIterator(_) => "ArrayIterator",
                 Self::Function(_) => "Function",
                 Self::RegExp(_) => "RegExp",
                 Self::Map(_) => "Map",
                 Self::String(_) => "String",
+                Self::StringIterator(_) => "StringIterator",
                 Self::Symbol(_) => "Symbol",
                 Self::Error => "Error",
                 Self::Ordinary => "Ordinary",
@@ -252,6 +259,36 @@ impl Object {
         }
     }
 
+    /// Checks if it is an `ArrayIterator` object.
+    #[inline]
+    pub fn is_array_iterator(&self) -> bool {
+        matches!(self.data, ObjectData::ArrayIterator(_))
+    }
+
+    #[inline]
+    pub fn as_array_iterator(&self) -> Option<&ArrayIterator> {
+        match self.data {
+            ObjectData::ArrayIterator(ref iter) => Some(iter),
+            _ => None,
+        }
+    }
+
+    #[inline]
+    pub fn as_array_iterator_mut(&mut self) -> Option<&mut ArrayIterator> {
+        match &mut self.data {
+            ObjectData::ArrayIterator(iter) => Some(iter),
+            _ => None,
+        }
+    }
+
+    #[inline]
+    pub fn as_string_iterator_mut(&mut self) -> Option<&mut StringIterator> {
+        match &mut self.data {
+            ObjectData::StringIterator(iter) => Some(iter),
+            _ => None,
+        }
+    }
+
     /// Checks if it is a `Map` object.pub
     #[inline]
     pub fn is_map(&self) -> bool {

boa/src/syntax/ast/keyword.rs

@@ -291,6 +291,16 @@ pub enum Keyword {
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new
     New,
 
+    /// The `of` keyword.
+    ///
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-for-in-and-for-of-statements
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/for...of
+    Of,
+
     /// The `return` keyword
     ///
     /// More information:
@@ -467,6 +477,7 @@ impl Keyword {
             Self::Import => "import",
             Self::Let => "let",
             Self::New => "new",
+            Self::Of => "of",
             Self::Return => "return",
             Self::Super => "super",
             Self::Switch => "switch",
@@ -538,6 +549,7 @@ impl FromStr for Keyword {
             "import" => Ok(Self::Import),
             "let" => Ok(Self::Let),
             "new" => Ok(Self::New),
+            "of" => Ok(Self::Of),
             "return" => Ok(Self::Return),
             "super" => Ok(Self::Super),
             "switch" => Ok(Self::Switch),

boa/src/syntax/ast/node/declaration/const_decl_list/mod.rs

@@ -38,7 +38,11 @@ pub struct ConstDeclList {
 impl Executable for ConstDeclList {
     fn run(&self, interpreter: &mut Context) -> Result<Value> {
         for decl in self.as_ref() {
-            let val = decl.init().run(interpreter)?;
+            let val = if let Some(init) = decl.init() {
+                init.run(interpreter)?
+            } else {
+                return interpreter.throw_syntax_error("missing = in const declaration");
+            };
 
             interpreter
                 .realm_mut()
@@ -99,25 +103,29 @@ impl From<ConstDeclList> for Node {
 #[derive(Clone, Debug, Trace, Finalize, PartialEq)]
 pub struct ConstDecl {
     name: Identifier,
-    init: Node,
+    init: Option<Node>,
 }
 
 impl fmt::Display for ConstDecl {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        write!(f, "{} = {}", self.name, self.init)
+        fmt::Display::fmt(&self.name, f)?;
+        if let Some(ref init) = self.init {
+            write!(f, " = {}", init)?;
+        }
+        Ok(())
     }
 }
 
 impl ConstDecl {
     /// Creates a new variable declaration.
-    pub(in crate::syntax) fn new<N, I>(name: N, init: I) -> Self
+    pub(in crate::syntax) fn new<N, I>(name: N, init: Option<I>) -> Self
     where
         N: Into<Identifier>,
         I: Into<Node>,
     {
         Self {
             name: name.into(),
-            init: init.into(),
+            init: init.map(|n| n.into()),
         }
     }
 
@@ -127,7 +135,7 @@ impl ConstDecl {
     }
 
     /// Gets the initialization node for the variable, if any.
-    pub fn init(&self) -> &Node {
+    pub fn init(&self) -> &Option<Node> {
         &self.init
     }
 }

boa/src/syntax/ast/node/iteration/for_of_loop/mod.rs

@@ -0,0 +1,208 @@
+use crate::{
+    builtins::iterable::get_iterator,
+    environment::lexical_environment::{new_declarative_environment, VariableScope},
+    exec::{Executable, InterpreterState},
+    syntax::ast::node::Node,
+    BoaProfiler, Context, Result, Value,
+};
+use gc::{Finalize, Trace};
+use std::fmt;
+
+#[cfg(feature = "serde")]
+use serde::{Deserialize, Serialize};
+
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+#[derive(Clone, Debug, Trace, Finalize, PartialEq)]
+pub struct ForOfLoop {
+    variable: Box<Node>,
+    iterable: Box<Node>,
+    body: Box<Node>,
+}
+
+impl ForOfLoop {
+    pub fn new<V, I, B>(variable: V, iterable: I, body: B) -> Self
+    where
+        V: Into<Node>,
+        I: Into<Node>,
+        B: Into<Node>,
+    {
+        Self {
+            variable: Box::new(variable.into()),
+            iterable: Box::new(iterable.into()),
+            body: Box::new(body.into()),
+        }
+    }
+
+    pub fn variable(&self) -> &Node {
+        &self.variable
+    }
+
+    pub fn iterable(&self) -> &Node {
+        &self.iterable
+    }
+
+    pub fn body(&self) -> &Node {
+        &self.body
+    }
+
+    pub fn display(&self, f: &mut fmt::Formatter<'_>, indentation: usize) -> fmt::Result {
+        write!(f, "for ({} of {}) {{", self.variable, self.iterable)?;
+        self.body().display(f, indentation + 1)?;
+        f.write_str("}")
+    }
+}
+
+impl fmt::Display for ForOfLoop {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.display(f, 0)
+    }
+}
+
+impl From<ForOfLoop> for Node {
+    fn from(for_of: ForOfLoop) -> Node {
+        Self::ForOfLoop(for_of)
+    }
+}
+
+impl Executable for ForOfLoop {
+    fn run(&self, interpreter: &mut Context) -> Result<Value> {
+        let _timer = BoaProfiler::global().start_event("ForOf", "exec");
+        let iterable = self.iterable().run(interpreter)?;
+        let iterator = get_iterator(interpreter, iterable)?;
+        let mut result = Value::undefined();
+
+        loop {
+            {
+                let env = &mut interpreter.realm_mut().environment;
+                env.push(new_declarative_environment(Some(
+                    env.get_current_environment_ref().clone(),
+                )));
+            }
+            let iterator_result = iterator.next(interpreter)?;
+            if iterator_result.is_done() {
+                break;
+            }
+            let next_result = iterator_result.value();
+
+            match self.variable() {
+                Node::Identifier(ref name) => {
+                    let environment = &mut interpreter.realm_mut().environment;
+
+                    if environment.has_binding(name.as_ref()) {
+                        // Binding already exists
+                        environment.set_mutable_binding(name.as_ref(), next_result.clone(), true);
+                    } else {
+                        environment.create_mutable_binding(
+                            name.as_ref().to_owned(),
+                            true,
+                            VariableScope::Function,
+                        );
+                        environment.initialize_binding(name.as_ref(), next_result.clone());
+                    }
+                }
+                Node::VarDeclList(ref list) => match list.as_ref() {
+                    [var] => {
+                        let environment = &mut interpreter.realm_mut().environment;
+
+                        if var.init().is_some() {
+                            return interpreter.throw_syntax_error("a declaration in the head of a for-of loop can't have an initializer");
+                        }
+
+                        if environment.has_binding(var.name()) {
+                            environment.set_mutable_binding(var.name(), next_result, true);
+                        } else {
+                            environment.create_mutable_binding(
+                                var.name().to_owned(),
+                                false,
+                                VariableScope::Function,
+                            );
+                            environment.initialize_binding(var.name(), next_result);
+                        }
+                    }
+                    _ => {
+                        return interpreter.throw_syntax_error(
+                            "only one variable can be declared in the head of a for-of loop",
+                        )
+                    }
+                },
+                Node::LetDeclList(ref list) => match list.as_ref() {
+                    [var] => {
+                        let environment = &mut interpreter.realm_mut().environment;
+
+                        if var.init().is_some() {
+                            return interpreter.throw_syntax_error("a declaration in the head of a for-of loop can't have an initializer");
+                        }
+
+                        environment.create_mutable_binding(
+                            var.name().to_owned(),
+                            false,
+                            VariableScope::Block,
+                        );
+                        environment.initialize_binding(var.name(), next_result);
+                    }
+                    _ => {
+                        return interpreter.throw_syntax_error(
+                            "only one variable can be declared in the head of a for-of loop",
+                        )
+                    }
+                },
+                Node::ConstDeclList(ref list) => match list.as_ref() {
+                    [var] => {
+                        let environment = &mut interpreter.realm_mut().environment;
+
+                        if var.init().is_some() {
+                            return interpreter.throw_syntax_error("a declaration in the head of a for-of loop can't have an initializer");
+                        }
+
+                        environment.create_immutable_binding(
+                            var.name().to_owned(),
+                            false,
+                            VariableScope::Block,
+                        );
+                        environment.initialize_binding(var.name(), next_result);
+                    }
+                    _ => {
+                        return interpreter.throw_syntax_error(
+                            "only one variable can be declared in the head of a for-of loop",
+                        )
+                    }
+                },
+                Node::Assign(_) => {
+                    return interpreter.throw_syntax_error(
+                        "a declaration in the head of a for-of loop can't have an initializer",
+                    );
+                }
+                _ => {
+                    return interpreter
+                        .throw_syntax_error("unknown left hand side in head of for-of loop")
+                }
+            }
+
+            result = self.body().run(interpreter)?;
+            match interpreter.executor().get_current_state() {
+                InterpreterState::Break(_label) => {
+                    // TODO break to label.
+
+                    // Loops 'consume' breaks.
+                    interpreter
+                        .executor()
+                        .set_current_state(InterpreterState::Executing);
+                    break;
+                }
+                InterpreterState::Continue(_label) => {
+                    // TODO continue to label.
+                    interpreter
+                        .executor()
+                        .set_current_state(InterpreterState::Executing);
+                    // after breaking out of the block, continue execution of the loop
+                }
+                InterpreterState::Return => return Ok(result),
+                InterpreterState::Executing => {
+                    // Continue execution.
+                }
+            }
+            let _ = interpreter.realm_mut().environment.pop();
+        }
+        Ok(result)
+    }
+}

boa/src/syntax/ast/node/iteration/mod.rs

@@ -1,7 +1,8 @@
 //! Iteration nodes
 
 pub use self::{
-    continue_node::Continue, do_while_loop::DoWhileLoop, for_loop::ForLoop, while_loop::WhileLoop,
+    continue_node::Continue, do_while_loop::DoWhileLoop, for_loop::ForLoop, for_of_loop::ForOfLoop,
+    while_loop::WhileLoop,
 };
 
 #[cfg(test)]
@@ -32,4 +33,5 @@ macro_rules! handle_state_with_labels {
 pub mod continue_node;
 pub mod do_while_loop;
 pub mod for_loop;
+pub mod for_of_loop;
 pub mod while_loop;

boa/src/syntax/ast/node/iteration/tests.rs

@@ -1,4 +1,4 @@
-use crate::exec;
+use crate::{exec, forward, Context};
 
 #[test]
 fn while_loop_late_break() {
@@ -192,6 +192,133 @@ fn do_while_loop_continue() {
     assert_eq!(&exec(scenario), "[ 1, 2 ]");
 }
 
+#[test]
+fn for_of_loop_declaration() {
+    let mut engine = Context::new();
+    let scenario = r#"
+        var result = 0;
+        for (i of [1, 2, 3]) {
+            result = i;
+        }
+    "#;
+    engine.eval(scenario).unwrap();
+    assert_eq!(&forward(&mut engine, "result"), "3");
+    assert_eq!(&forward(&mut engine, "i"), "3");
+}
+
+#[test]
+fn for_of_loop_var() {
+    let mut engine = Context::new();
+    let scenario = r#"
+        var result = 0;
+        for (var i of [1, 2, 3]) {
+            result = i;
+        }
+    "#;
+    engine.eval(scenario).unwrap();
+    assert_eq!(&forward(&mut engine, "result"), "3");
+    assert_eq!(&forward(&mut engine, "i"), "3");
+}
+
+#[test]
+fn for_of_loop_let() {
+    let mut engine = Context::new();
+    let scenario = r#"
+        var result = 0;
+        for (let i of [1, 2, 3]) {
+            result = i;
+        }
+    "#;
+    engine.eval(scenario).unwrap();
+    assert_eq!(&forward(&mut engine, "result"), "3");
+    assert_eq!(
+        &forward(
+            &mut engine,
+            r#"
+        try {
+            i
+        } catch(e) {
+            e.toString()
+        }
+    "#
+        ),
+        "\"ReferenceError: i is not defined\""
+    );
+}
+
+#[test]
+fn for_of_loop_const() {
+    let mut engine = Context::new();
+    let scenario = r#"
+        var result = 0;
+        for (let i of [1, 2, 3]) {
+            result = i;
+        }
+    "#;
+    engine.eval(scenario).unwrap();
+    assert_eq!(&forward(&mut engine, "result"), "3");
+    assert_eq!(
+        &forward(
+            &mut engine,
+            r#"
+        try {
+            i
+        } catch(e) {
+            e.toString()
+        }
+    "#
+        ),
+        "\"ReferenceError: i is not defined\""
+    );
+}
+
+#[test]
+fn for_of_loop_break() {
+    let mut engine = Context::new();
+    let scenario = r#"
+        var result = 0;
+        for (var i of [1, 2, 3]) {
+            if (i > 1)
+                break;
+            result = i
+        }
+    "#;
+    engine.eval(scenario).unwrap();
+    assert_eq!(&forward(&mut engine, "result"), "1");
+    assert_eq!(&forward(&mut engine, "i"), "2");
+}
+
+#[test]
+fn for_of_loop_continue() {
+    let mut engine = Context::new();
+    let scenario = r#"
+        var result = 0;
+        for (var i of [1, 2, 3]) {
+            if (i == 3)
+                continue;
+            result = i
+        }
+    "#;
+    engine.eval(scenario).unwrap();
+    assert_eq!(&forward(&mut engine, "result"), "2");
+    assert_eq!(&forward(&mut engine, "i"), "3");
+}
+
+#[test]
+fn for_of_loop_return() {
+    let mut engine = Context::new();
+    let scenario = r#"
+        function foo() {
+            for (i of [1, 2, 3]) {
+                if (i > 1)
+                    return i;
+            }
+        }
+    "#;
+    engine.eval(scenario).unwrap();
+    assert_eq!(&forward(&mut engine, "foo()"), "2");
+}
+
 #[test]
 fn for_loop_break_label() {
     let scenario = r#"

boa/src/syntax/ast/node/mod.rs

@@ -30,7 +30,7 @@ pub use self::{
     expression::{Call, New},
     field::{GetConstField, GetField},
     identifier::Identifier,
-    iteration::{Continue, DoWhileLoop, ForLoop, WhileLoop},
+    iteration::{Continue, DoWhileLoop, ForLoop, ForOfLoop, WhileLoop},
     object::Object,
     operator::{Assign, BinOp, UnaryOp},
     return_smt::Return,
@@ -114,6 +114,9 @@ pub enum Node {
     /// A `for` statement. [More information](./iteration/struct.ForLoop.html).
     ForLoop(ForLoop),
 
+    /// A `for...of` statement. [More information](./iteration/struct.ForOf.html).
+    ForOfLoop(ForOfLoop),
+
     /// An 'if' statement. [More information](./conditional/struct.If.html).
     If(If),
 
@@ -209,6 +212,7 @@ impl Node {
             Self::Const(ref c) => write!(f, "{}", c),
             Self::ConditionalOp(ref cond_op) => Display::fmt(cond_op, f),
             Self::ForLoop(ref for_loop) => for_loop.display(f, indentation),
+            Self::ForOfLoop(ref for_of) => for_of.display(f, indentation),
             Self::This => write!(f, "this"),
             Self::Try(ref try_catch) => try_catch.display(f, indentation),
             Self::Break(ref break_smt) => Display::fmt(break_smt, f),
@@ -263,6 +267,7 @@ impl Executable for Node {
             Node::WhileLoop(ref while_loop) => while_loop.run(interpreter),
             Node::DoWhileLoop(ref do_while) => do_while.run(interpreter),
             Node::ForLoop(ref for_loop) => for_loop.run(interpreter),
+            Node::ForOfLoop(ref for_of_loop) => for_of_loop.run(interpreter),
             Node::If(ref if_smt) => if_smt.run(interpreter),
             Node::ConditionalOp(ref op) => op.run(interpreter),
             Node::Switch(ref switch) => switch.run(interpreter),

boa/src/syntax/parser/expression/primary/object_initializer/tests.rs

@@ -23,9 +23,11 @@ fn check_object_literal() {
             b: false,
         };
         ",
-        vec![
-            ConstDeclList::from(vec![ConstDecl::new("x", Object::from(object_properties))]).into(),
-        ],
+        vec![ConstDeclList::from(vec![ConstDecl::new(
+            "x",
+            Some(Object::from(object_properties)),
+        )])
+        .into()],
     );
 }
 
@@ -47,9 +49,11 @@ fn check_object_short_function() {
             b() {},
         };
         ",
-        vec![
-            ConstDeclList::from(vec![ConstDecl::new("x", Object::from(object_properties))]).into(),
-        ],
+        vec![ConstDeclList::from(vec![ConstDecl::new(
+            "x",
+            Some(Object::from(object_properties)),
+        )])
+        .into()],
     );
 }
 
@@ -75,9 +79,11 @@ fn check_object_short_function_arguments() {
             b(test) {}
          };
         ",
-        vec![
-            ConstDeclList::from(vec![ConstDecl::new("x", Object::from(object_properties))]).into(),
-        ],
+        vec![ConstDeclList::from(vec![ConstDecl::new(
+            "x",
+            Some(Object::from(object_properties)),
+        )])
+        .into()],
     );
 }
 
@@ -98,9 +104,11 @@ fn check_object_getter() {
             get b() {}
         };
         ",
-        vec![
-            ConstDeclList::from(vec![ConstDecl::new("x", Object::from(object_properties))]).into(),
-        ],
+        vec![ConstDeclList::from(vec![ConstDecl::new(
+            "x",
+            Some(Object::from(object_properties)),
+        )])
+        .into()],
     );
 }
 
@@ -125,8 +133,10 @@ fn check_object_setter() {
             set b(test) {}
         };
         ",
-        vec![
-            ConstDeclList::from(vec![ConstDecl::new("x", Object::from(object_properties))]).into(),
-        ],
+        vec![ConstDeclList::from(vec![ConstDecl::new(
+            "x",
+            Some(Object::from(object_properties)),
+        )])
+        .into()],
     );
 }

boa/src/syntax/parser/statement/declaration/lexical.rs

@@ -37,11 +37,17 @@ pub(super) struct LexicalDeclaration {
     allow_in: AllowIn,
     allow_yield: AllowYield,
     allow_await: AllowAwait,
+    const_init_required: bool,
 }
 
 impl LexicalDeclaration {
     /// Creates a new `LexicalDeclaration` parser.
-    pub(super) fn new<I, Y, A>(allow_in: I, allow_yield: Y, allow_await: A) -> Self
+    pub(super) fn new<I, Y, A>(
+        allow_in: I,
+        allow_yield: Y,
+        allow_await: A,
+        const_init_required: bool,
+    ) -> Self
     where
         I: Into<AllowIn>,
         Y: Into<AllowYield>,
@@ -51,6 +57,7 @@ impl LexicalDeclaration {
             allow_in: allow_in.into(),
             allow_yield: allow_yield.into(),
             allow_await: allow_await.into(),
+            const_init_required,
         }
     }
 }
@@ -66,14 +73,22 @@ where
         let tok = cursor.next()?.ok_or(ParseError::AbruptEnd)?;
 
         match tok.kind() {
-            TokenKind::Keyword(Keyword::Const) => {
-                BindingList::new(self.allow_in, self.allow_yield, self.allow_await, true)
-                    .parse(cursor)
-            }
-            TokenKind::Keyword(Keyword::Let) => {
-                BindingList::new(self.allow_in, self.allow_yield, self.allow_await, false)
-                    .parse(cursor)
-            }
+            TokenKind::Keyword(Keyword::Const) => BindingList::new(
+                self.allow_in,
+                self.allow_yield,
+                self.allow_await,
+                true,
+                self.const_init_required,
+            )
+            .parse(cursor),
+            TokenKind::Keyword(Keyword::Let) => BindingList::new(
+                self.allow_in,
+                self.allow_yield,
+                self.allow_await,
+                false,
+                self.const_init_required,
+            )
+            .parse(cursor),
             _ => unreachable!("unknown token found: {:?}", tok),
         }
     }
@@ -94,11 +109,18 @@ struct BindingList {
     allow_yield: AllowYield,
     allow_await: AllowAwait,
     is_const: bool,
+    const_init_required: bool,
 }
 
 impl BindingList {
     /// Creates a new `BindingList` parser.
-    fn new<I, Y, A>(allow_in: I, allow_yield: Y, allow_await: A, is_const: bool) -> Self
+    fn new<I, Y, A>(
+        allow_in: I,
+        allow_yield: Y,
+        allow_await: A,
+        is_const: bool,
+        const_init_required: bool,
+    ) -> Self
     where
         I: Into<AllowIn>,
         Y: Into<AllowYield>,
@@ -109,6 +131,7 @@ impl BindingList {
             allow_yield: allow_yield.into(),
             allow_await: allow_await.into(),
             is_const,
+            const_init_required,
         }
     }
 }
@@ -133,14 +156,18 @@ where
                     .parse(cursor)?;
 
             if self.is_const {
-                if let Some(init) = init {
-                    const_decls.push(ConstDecl::new(ident, init));
+                if self.const_init_required {
+                    if init.is_some() {
+                        const_decls.push(ConstDecl::new(ident, init));
+                    } else {
+                        return Err(ParseError::expected(
+                            vec![TokenKind::Punctuator(Punctuator::Assign)],
+                            cursor.next()?.ok_or(ParseError::AbruptEnd)?,
+                            "const declaration",
+                        ));
+                    }
                 } else {
-                    return Err(ParseError::expected(
-                        vec![TokenKind::Punctuator(Punctuator::Assign)],
-                        cursor.next()?.ok_or(ParseError::AbruptEnd)?,
-                        "const declaration",
-                    ));
+                    const_decls.push(ConstDecl::new(ident, init))
                 }
             } else {
                 let_decls.push(LetDecl::new(ident, init));
@@ -148,6 +175,12 @@ where
 
             match cursor.peek_semicolon()? {
                 SemicolonResult::Found(_) => break,
+                SemicolonResult::NotFound(tk)
+                    if tk.kind() == &TokenKind::Keyword(Keyword::Of)
+                        || tk.kind() == &TokenKind::Keyword(Keyword::In) =>
+                {
+                    break
+                }
                 SemicolonResult::NotFound(tk)
                     if tk.kind() == &TokenKind::Punctuator(Punctuator::Comma) =>
                 {

boa/src/syntax/parser/statement/declaration/mod.rs

@@ -35,10 +35,11 @@ use std::io::Read;
 pub(super) struct Declaration {
     allow_yield: AllowYield,
     allow_await: AllowAwait,
+    const_init_required: bool,
 }
 
 impl Declaration {
-    pub(super) fn new<Y, A>(allow_yield: Y, allow_await: A) -> Self
+    pub(super) fn new<Y, A>(allow_yield: Y, allow_await: A, const_init_required: bool) -> Self
     where
         Y: Into<AllowYield>,
         A: Into<AllowAwait>,
@@ -46,6 +47,7 @@ impl Declaration {
         Self {
             allow_yield: allow_yield.into(),
             allow_await: allow_await.into(),
+            const_init_required,
         }
     }
 }
@@ -65,7 +67,13 @@ where
                 HoistableDeclaration::new(self.allow_yield, self.allow_await, false).parse(cursor)
             }
             TokenKind::Keyword(Keyword::Const) | TokenKind::Keyword(Keyword::Let) => {
-                LexicalDeclaration::new(true, self.allow_yield, self.allow_await).parse(cursor)
+                LexicalDeclaration::new(
+                    true,
+                    self.allow_yield,
+                    self.allow_await,
+                    self.const_init_required,
+                )
+                .parse(cursor)
             }
             _ => unreachable!("unknown token found: {:?}", tok),
         }

boa/src/syntax/parser/statement/declaration/tests.rs

@@ -124,7 +124,7 @@ fn multiple_let_declaration() {
 fn const_declaration() {
     check_parser(
         "const a = 5;",
-        vec![ConstDeclList::from(ConstDecl::new("a", Const::from(5))).into()],
+        vec![ConstDeclList::from(ConstDecl::new("a", Some(Const::from(5)))).into()],
     );
 }
 
@@ -133,12 +133,12 @@ fn const_declaration() {
 fn const_declaration_keywords() {
     check_parser(
         "const yield = 5;",
-        vec![ConstDeclList::from(ConstDecl::new("yield", Const::from(5))).into()],
+        vec![ConstDeclList::from(ConstDecl::new("yield", Some(Const::from(5)))).into()],
     );
 
     check_parser(
         "const await = 5;",
-        vec![ConstDeclList::from(ConstDecl::new("await", Const::from(5))).into()],
+        vec![ConstDeclList::from(ConstDecl::new("await", Some(Const::from(5)))).into()],
     );
 }
 
@@ -147,7 +147,7 @@ fn const_declaration_keywords() {
 fn const_declaration_no_spaces() {
     check_parser(
         "const a=5;",
-        vec![ConstDeclList::from(ConstDecl::new("a", Const::from(5))).into()],
+        vec![ConstDeclList::from(ConstDecl::new("a", Some(Const::from(5)))).into()],
     );
 }
 
@@ -163,8 +163,8 @@ fn multiple_const_declaration() {
     check_parser(
         "const a = 5, c = 6;",
         vec![ConstDeclList::from(vec![
-            ConstDecl::new("a", Const::from(5)),
-            ConstDecl::new("c", Const::from(6)),
+            ConstDecl::new("a", Some(Const::from(5))),
+            ConstDecl::new("c", Some(Const::from(6))),
         ])
         .into()],
     );

boa/src/syntax/parser/statement/iteration/for_statement.rs

@@ -11,7 +11,7 @@ use crate::syntax::lexer::TokenKind;
 use crate::{
     syntax::{
         ast::{
-            node::{ForLoop, Node},
+            node::{ForLoop, ForOfLoop, Node},
             Const, Keyword, Punctuator,
         },
         parser::{
@@ -65,7 +65,7 @@ impl<R> TokenParser<R> for ForStatement
 where
     R: Read,
 {
-    type Output = ForLoop;
+    type Output = Node;
 
     fn parse(self, cursor: &mut Cursor<R>) -> Result<Self::Output, ParseError> {
         let _timer = BoaProfiler::global().start_event("ForStatement", "Parsing");
@@ -82,7 +82,7 @@ where
                 )
             }
             TokenKind::Keyword(Keyword::Let) | TokenKind::Keyword(Keyword::Const) => {
-                Some(Declaration::new(self.allow_yield, self.allow_await).parse(cursor)?)
+                Some(Declaration::new(self.allow_yield, self.allow_await, false).parse(cursor)?)
             }
             TokenKind::Punctuator(Punctuator::Semicolon) => None,
             _ => Some(Expression::new(true, self.allow_yield, self.allow_await).parse(cursor)?),
@@ -93,8 +93,14 @@ where
             Some(tok) if tok.kind() == &TokenKind::Keyword(Keyword::In) => {
                 unimplemented!("for...in statement")
             }
-            Some(tok) if tok.kind() == &TokenKind::identifier("of") => {
-                unimplemented!("for...of statement")
+            Some(tok) if tok.kind() == &TokenKind::Keyword(Keyword::Of) && init.is_some() => {
+                let _ = cursor.next();
+                let iterable =
+                    Expression::new(true, self.allow_yield, self.allow_await).parse(cursor)?;
+                cursor.expect(Punctuator::CloseParen, "for of statement")?;
+                let body = Statement::new(self.allow_yield, self.allow_await, self.allow_return)
+                    .parse(cursor)?;
+                return Ok(ForOfLoop::new(init.unwrap(), iterable, body).into());
             }
             _ => {}
         }
@@ -124,6 +130,6 @@ where
             Statement::new(self.allow_yield, self.allow_await, self.allow_return).parse(cursor)?;
 
         // TODO: do not encapsulate the `for` in a block just to have an inner scope.
-        Ok(ForLoop::new(init, cond, step, body))
+        Ok(ForLoop::new(init, cond, step, body).into())
     }
 }

boa/src/syntax/parser/statement/mod.rs

@@ -38,9 +38,11 @@ use self::{
 
 use super::{AllowAwait, AllowReturn, AllowYield, Cursor, ParseError, TokenParser};
 
-use crate::syntax::lexer::{InputElement, TokenKind};
 use crate::{
-    syntax::ast::{node, Keyword, Node, Punctuator},
+    syntax::{
+        ast::{node, Keyword, Node, Punctuator},
+        lexer::{InputElement, TokenKind},
+    },
     BoaProfiler,
 };
 use labelled_stm::LabelledStatement;
@@ -373,7 +375,7 @@ where
             TokenKind::Keyword(Keyword::Function)
             | TokenKind::Keyword(Keyword::Const)
             | TokenKind::Keyword(Keyword::Let) => {
-                Declaration::new(self.allow_yield, self.allow_await).parse(cursor)
+                Declaration::new(self.allow_yield, self.allow_await, true).parse(cursor)
             }
             _ => {
                 Statement::new(self.allow_yield, self.allow_await, self.allow_return).parse(cursor)

boa/src/value/mod.rs

@@ -303,6 +303,14 @@ impl Value {
         }
     }
 
+    #[inline]
+    pub fn as_gc_object(&self) -> Option<GcObject> {
+        match self {
+            Self::Object(o) => Some(o.clone()),
+            _ => None,
+        }
+    }
+
     #[inline]
     pub fn as_object_mut(&self) -> Option<GcCellRefMut<'_, Object>> {
         match *self {
@@ -317,6 +325,13 @@ impl Value {
         matches!(self, Self::Symbol(_))
     }
 
+    pub fn as_symbol(&self) -> Option<RcSymbol> {
+        match self {
+            Self::Symbol(symbol) => Some(symbol.clone()),
+            _ => None,
+        }
+    }
+
     /// Returns true if the value is a function
     #[inline]
     pub fn is_function(&self) -> bool {
@@ -398,6 +413,14 @@ impl Value {
         matches!(self, Self::Boolean(_))
     }
 
+    #[inline]
+    pub fn as_boolean(&self) -> Option<bool> {
+        match self {
+            Self::Boolean(boolean) => Some(*boolean),
+            _ => None,
+        }
+    }
+
     /// Returns true if the value is a bigint.
     #[inline]
     pub fn is_bigint(&self) -> bool {
@@ -916,7 +939,7 @@ impl Value {
     /// [table]: https://tc39.es/ecma262/#table-14
     /// [spec]: https://tc39.es/ecma262/#sec-requireobjectcoercible
     #[inline]
-    pub fn require_object_coercible<'a>(&'a self, ctx: &mut Context) -> Result<&'a Value> {
+    pub fn require_object_coercible(&self, ctx: &mut Context) -> Result<&Value> {
         if self.is_null_or_undefined() {
             Err(ctx.construct_type_error("cannot convert null or undefined to Object"))
         } else {