Why does (x += x += 1) evaluate differently in C and Javascript?

Question

If the value of the variable x is initially 0, the expression x += x += 1 will evaluate to 2 in C, and to 1 in Javascript.

The semantics for C seems obvious to me: x += x += 1 is interpreted as x += (x += 1) which is, in turn, equivalent to

x += 1 x += x  // where x is 1 at this point 

What is the logic behind Javascript's interpretation? What specification enforces such behaviour? (It should be noted, by the way, that Java agrees with Javascript here).

Update: It turns out the expression x += x += 1 has undefined behaviour according to the C standard (thanks ouah, John Bode, DarkDust, Drew Dormann), which seems to spoil the whole point of the question for some readers. The expression can be made standards-compliant by inserting an identity function into it as follows: x += id(x += 1). The same modification can be made to the Javascript code and the question still remains as stated. Presuming that the majority of the readers can understand the point behind "non-standards-compliant" formulation I'll keep it as it is more concise.

Update 2: It turns out that according to C99 the introduction of the identity function is probably not solving the ambiguity. In this case, dear reader, please regard the original question as pertaining to C++ rather than C99, where "+=" can be most probably now safely be regarded as an overloadable operator with a uniquely defined sequence of operations. That is, x += x += 1 is now equivalent to operator+=(x, operator+=(x, 1)). Sorry for the long road to standards-compliance.

Answer 1

x += x += 1; is undefined behavior in C.

The expression statement violates sequence points rules.

(C99, 6.5p2) "Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression."