Make `String.prototype.repeat()` ECMAScript specification compliant (#582)

boa/src/builtins/string/mod.rs

@@ -41,6 +41,13 @@ impl String {
     /// The amount of arguments this function object takes.
     pub(crate) const LENGTH: usize = 1;
 
+    ///  JavaScript strings must be between `0` and less than positive `Infinity` and cannot be a negative number.
+    /// The range of allowed values can be described like this: `[0, +∞)`.
+    ///
+    /// The resulting string can also not be larger than the maximum string size,
+    /// which can differ in JavaScript engines. In Boa it is `2^32 - 1`
+    pub(crate) const MAX_STRING_LENGTH: f64 = u32::MAX as f64;
+
     fn this_string_value(this: &Value, ctx: &mut Interpreter) -> Result<RcString, Value> {
         match this {
             Value::String(ref string) => return Ok(string.clone()),
@@ -206,16 +213,27 @@ impl String {
     /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.repeat
     /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat
     pub(crate) fn repeat(this: &Value, args: &[Value], ctx: &mut Interpreter) -> ResultValue {
-        // First we get it the actual string a private field stored on the object only the engine has access to.
-        // Then we convert it into a Rust String by wrapping it in from_value
-        let primitive_val = ctx.to_string(this)?;
+        let object = ctx.require_object_coercible(this)?;
+        let string = ctx.to_string(object)?;
 
-        let repeat_times = usize::from(
-            args.get(0)
-                .expect("failed to get argument for String method"),
-        );
+        if let Some(arg) = args.get(0) {
+            let n = ctx.to_integer(arg)?;
+            if n < 0.0 {
+                return ctx.throw_range_error("repeat count cannot be a negative number");
+            }
+
+            if n.is_infinite() {
+                return ctx.throw_range_error("repeat count cannot be infinity");
+            }
 
-        Ok(Value::from(primitive_val.repeat(repeat_times)))
+            if n * (string.len() as f64) > Self::MAX_STRING_LENGTH {
+                return ctx
+                    .throw_range_error("repeat count must not overflow maximum string length");
+            }
+            Ok(string.repeat(n as usize).into())
+        } else {
+            Ok("".into())
+        }
     }
 
     /// `String.prototype.slice( beginIndex [, endIndex] )`

boa/src/builtins/string/tests.rs

@@ -128,6 +128,81 @@ fn repeat() {
     assert_eq!(forward(&mut engine, "zh.repeat(2)"), "中文中文");
 }
 
+#[test]
+fn repeat_throws_when_count_is_negative() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+
+    assert_eq!(
+        forward(
+            &mut engine,
+            r#"
+        try {
+            'x'.repeat(-1)
+        } catch (e) {
+            e.toString()
+        }
+    "#
+        ),
+        "RangeError: repeat count cannot be a negative number"
+    );
+}
+
+#[test]
+fn repeat_throws_when_count_is_infinity() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+
+    assert_eq!(
+        forward(
+            &mut engine,
+            r#"
+        try {
+            'x'.repeat(Infinity)
+        } catch (e) {
+            e.toString()
+        }
+    "#
+        ),
+        "RangeError: repeat count cannot be infinity"
+    );
+}
+
+#[test]
+fn repeat_throws_when_count_overflows_max_length() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+
+    assert_eq!(
+        forward(
+            &mut engine,
+            r#"
+        try {
+            'x'.repeat(2 ** 64)
+        } catch (e) {
+            e.toString()
+        }
+    "#
+        ),
+        "RangeError: repeat count must not overflow maximum string length"
+    );
+}
+
+#[test]
+fn repeat_generic() {
+    let realm = Realm::create();
+    let mut engine = Interpreter::new(realm);
+    let init = "Number.prototype.repeat = String.prototype.repeat;";
+
+    forward(&mut engine, init);
+
+    assert_eq!(forward(&mut engine, "(0).repeat(0)"), "");
+    assert_eq!(forward(&mut engine, "(1).repeat(1)"), "1");
+
+    assert_eq!(forward(&mut engine, "(1).repeat(5)"), "11111");
+    assert_eq!(forward(&mut engine, "(12).repeat(3)"), "121212");
+}
+
 #[test]
 fn replace() {
     let realm = Realm::create();