Chaining compound assignment operators in JavaScript

Question

In C#,

string s = "abc";
s += (s += s);
Console.WriteLine(s);

writes abcabcabc (http://ideone.com/pFNFX2). This is fine, because the C# specification explicitly says in section 7.16.2 that

the operation is evaluated as x = x op y, except that x is evaluated only once.

However, in reading the description of the compound assignment operator in section 11.3.2 of the ECMAScript 5.1 Language Specification, I see no such "only once" qualifier in the semantics of this operator. Instead, all I see is:

1. Let lref be the result of evaluating LeftHandSideExpression.
2. Let lval be GetValue(lref).
3. Let rref be the result of evaluating AssignmentExpression.
4. Let rval be GetValue(rref).
5. Let r be the result of applying operator @ to lval and rval.
6. Throw a SyntaxError exception if the following conditions are all true: (snipped)
7. Call PutValue(lref, r).
8. Return r.

Therefore, it would appear (to me, anyway) that the following JavaScript code

var s = "abc";
s += (s += s);
alert(s);

would alert abcabcabcabc (because of the PutValue on line 7 in the parenthesized expression), but, in Chrome 22 anyway, it alerts abcabcabc.

So my question is: Am I misreading the specification, or is Chrome (V8 perhaps?) doing its own thing off-spec, so to speak?

Answer 1

So I think if you break open the operation, you have:

s += (s += s);

s = s + (s += s);
s = s + (s = s + s);    // Can't actually have the "s = " part, it's really just "s + s"
s = s + (s + s);
s = s + s + s;

Which means the result should be "abcabcabc".