Implement Array.prototype.reduceRight (#579)

boa/src/builtins/array/mod.rs

@@ -126,9 +126,9 @@ impl Array {
             1 if args[0].is_integer() => {
                 length = i32::from(&args[0]);
                 // TODO: It should not create an array of undefineds, but an empty array ("holy" array in V8) with length `n`.
-                // for n in 0..length {
-                //     this.set_field(n.to_string(), Value::undefined());
-                // }
+                for n in 0..length {
+                    this.set_field(n.to_string(), Value::undefined());
+                }
             }
             1 if args[0].is_double() => {
                 return ctx.throw_range_error("invalid array length");
@@ -1022,6 +1022,96 @@ impl Array {
         Ok(accumulator)
     }
 
+    /// `Array.prototype.reduceRight( callbackFn [ , initialValue ] )`
+    ///
+    /// The reduceRight method traverses right to left starting from the last defined value in the array,
+    /// accumulating a value using a given callback function. It returns the accumulated value.
+    ///  
+    /// More information:
+    ///  - [ECMAScript reference][spec]
+    ///  - [MDN documentation][mdn]
+    ///
+    /// [spec]: https://tc39.es/ecma262/#sec-array.prototype.reduceright
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduceRight
+    pub(crate) fn reduce_right(
+        this: &Value,
+        args: &[Value],
+        interpreter: &mut Interpreter,
+    ) -> ResultValue {
+        let this = interpreter.to_object(this)?;
+        let callback = match args.get(0) {
+            Some(value) if value.is_function() => value,
+            _ => return interpreter.throw_type_error("reduceRight was called without a callback"),
+        };
+        let initial_value = args.get(1).cloned().unwrap_or_else(Value::undefined);
+        let mut length = interpreter.to_length(&this.get_field("length"))?;
+        if length == 0 {
+            if initial_value.is_undefined() {
+                return interpreter.throw_type_error(
+                    "reduceRight was called on an empty array and with no initial value",
+                );
+            } else {
+                // early return to prevent usize subtraction errors
+                return Ok(initial_value);
+            }
+        }
+        let mut k = length - 1;
+        let mut accumulator = if initial_value.is_undefined() {
+            let mut k_present = false;
+            loop {
+                if this.has_field(&k.to_string()) {
+                    k_present = true;
+                    break;
+                }
+                // check must be done at the end to prevent usize subtraction error
+                if k == 0 {
+                    break;
+                }
+                k -= 1;
+            }
+            if !k_present {
+                return interpreter.throw_type_error(
+                    "reduceRight was called on an empty array and with no initial value",
+                );
+            }
+            let result = this.get_field(k.to_string());
+            k -= 1;
+            result
+        } else {
+            initial_value
+        };
+        loop {
+            if this.has_field(&k.to_string()) {
+                let arguments = [
+                    accumulator,
+                    this.get_field(k.to_string()),
+                    Value::from(k),
+                    this.clone(),
+                ];
+                accumulator = interpreter.call(&callback, &Value::undefined(), &arguments)?;
+                /* We keep track of possibly shortened length in order to prevent unnecessary iteration.
+                It may also be necessary to do this since shortening the array length does not
+                delete array elements. See: https://github.com/boa-dev/boa/issues/557 */
+                length = min(length, interpreter.to_length(&this.get_field("length"))?);
+
+                // move k to the last defined element if necessary or return if the length was set to 0
+                if k >= length {
+                    if length == 0 {
+                        return Ok(accumulator);
+                    } else {
+                        k = length - 1;
+                        continue;
+                    }
+                }
+            }
+            if k == 0 {
+                break;
+            }
+            k -= 1;
+        }
+        Ok(accumulator)
+    }
+
     /// Initialise the `Array` object on the global object.
     #[inline]
     pub(crate) fn init(global: &Value) -> (&str, Value) {
@@ -1054,6 +1144,7 @@ impl Array {
         make_builtin_fn(Self::slice, "slice", &prototype, 2);
         make_builtin_fn(Self::some, "some", &prototype, 2);
         make_builtin_fn(Self::reduce, "reduce", &prototype, 2);
+        make_builtin_fn(Self::reduce_right, "reduceRight", &prototype, 2);
 
         let array = make_constructor_fn(
             Self::NAME,

boa/src/builtins/array/tests.rs

@@ -899,6 +899,129 @@ fn reduce() {
     assert_eq!(result, "Reduce was called without a callback");
 }
 
+#[test]
+fn reduce_right() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+
+    let init = r#"
+        var arr = [1, 2, 3, 4];
+        function sub(acc, x) {
+            return acc - x;
+        }
+
+        function subIdx(acc, _, idx) {
+            return acc - idx;
+        }
+
+        function subLen(acc, _x, _idx, arr) {
+            return acc - arr.length;
+        }
+
+        function subResize(acc, x, idx, arr) {
+            if(idx == arr.length - 1) {
+                arr.length = 1;
+            }
+            return acc - x;
+        }
+        function subResize0(acc, x, idx, arr) {
+            if(idx == arr.length - 2) {
+                arr.length = 0;
+            }
+            return acc - x;
+        }
+        var delArray = [1, 2, 3, 4, 5];
+        delete delArray[0];
+        delete delArray[1];
+        delete delArray[3];
+
+    "#;
+    forward(&mut engine, init);
+
+    // empty array
+    let result = forward(&mut engine, "[].reduceRight(sub, 0)");
+    assert_eq!(result, "0");
+
+    // simple with initial value
+    let result = forward(&mut engine, "arr.reduceRight(sub, 0)");
+    assert_eq!(result, "-10");
+
+    // without initial value
+    let result = forward(&mut engine, "arr.reduceRight(sub)");
+    assert_eq!(result, "-2");
+
+    // with some items missing
+    let result = forward(&mut engine, "delArray.reduceRight(sub, 0)");
+    assert_eq!(result, "-8");
+
+    // with index
+    let result = forward(&mut engine, "arr.reduceRight(subIdx)");
+    assert_eq!(result, "1");
+
+    // with array
+    let result = forward(&mut engine, "arr.reduceRight(subLen)");
+    assert_eq!(result, "-8");
+
+    // resizing the array as reduce progresses
+    let result = forward(&mut engine, "arr.reduceRight(subResize, 0)");
+    assert_eq!(result, "-5");
+
+    // reset array
+    forward(&mut engine, "arr = [1, 2, 3, 4];");
+
+    // resizing the array to 0 as reduce progresses
+    let result = forward(&mut engine, "arr.reduceRight(subResize0, 0)");
+    assert_eq!(result, "-7");
+
+    // Empty array
+    let result = forward(
+        &mut engine,
+        r#"
+        try {
+            [].reduceRight((acc, x) => acc + x);
+        } catch(e) {
+            e.message
+        }
+    "#,
+    );
+    assert_eq!(
+        result,
+        "reduceRight was called on an empty array and with no initial value"
+    );
+
+    // Array with no defined elements
+    let result = forward(
+        &mut engine,
+        r#"
+        try {
+            var arr = [0, 1];
+            delete arr[0];
+            delete arr[1];
+            arr.reduceRight((acc, x) => acc + x);
+        } catch(e) {
+            e.message
+        }
+    "#,
+    );
+    assert_eq!(
+        result,
+        "reduceRight was called on an empty array and with no initial value"
+    );
+
+    // No callback
+    let result = forward(
+        &mut engine,
+        r#"
+        try {
+            arr.reduceRight("");
+        } catch(e) {
+            e.message
+        }
+    "#,
+    );
+    assert_eq!(result, "reduceRight was called without a callback");
+}
+
 #[test]
 fn call_array_constructor_with_one_argument() {
     let realm = Realm::create();