Question

Why does for ([] in object); work fine but [void 0 for ([] in object)] or (void 0 for ([] in object)) throw a syntax error for invalid left-hand assignment?

For example, I would expect the following code to work, but it doesn’t (the assertion isn’t even done due to the syntax error):

let (
  i = 0,
  iterable = {__iterator__:function() { var i = 5; while (i--) yield i; } }
) {
  for ([] in iterable) i++;
  console.assertEquals([void 0 for ([] in iterable)].length, i);
}

Answer 1

I did a little digging in jsparse.c of SpiderMonkey (which I assume is the JS parser you’re using for 1.8 features?)

The [code for (... in ...)] format or generator expression uses a different parse function than the standard for ([] in obj) uses.

Your LHS error is being created here:
(jsparse.c line 4200)

4194           case TOK_LB:
4195           case TOK_LC:
4196             pn3 = DestructuringExpr(cx, &data, tc, tt);
4197             if (!pn3)
4198                 return NULL;
4199 
4200             if (pn3->pn_type != TOK_RB || pn3->pn_count != 2) {
4201                 js_ReportCompileErrorNumber(cx, ts, NULL, JSREPORT_ERROR,
4202                                             JSMSG_BAD_FOR_LEFTSIDE);
4203                 return NULL;
4204             }

When it sees the [ it finds the Destructuring Expression, and ensures the count of the parser node is at 2.

Interestingly enough [void 0 for ([a,b] in iterator)] should work, although for reasons I don’t care to go digging for, the b from [a,b] is always undefined:

js> [[l1,typeof l2] for ([l1,l2] in {a:1, b:2})]
a,undefined,b,undefined

---

For reference – The standard for([] in {}) uses the following logic to determine the LHS validity:

2775 #if JS_HAS_DESTRUCTURING
2776                    ((JSVERSION_NUMBER(cx) == JSVERSION_1_7 &&
2777                      pn->pn_op == JSOP_FORIN)
2778                     ? (pn1->pn_type != TOK_RB || pn1->pn_count != 2)
2779                     : (pn1->pn_type != TOK_RB && pn1->pn_type != TOK_RC)) &&
2780 #endif

Which seems to mean that versions other than 1.7 don’t require 2 left hand values for this syntax. The generator expression might be using older parsing logic. This might be worth submitting as a report to the SpiderMonkey maintainers.