Javascript object initialization and evaluation order [duplicate]

Question

If I write

var a = [1,2];
var b = {
  foo: a.pop(),
  bar: a.pop()
};

What is the value of b, according to the specification?

(By experiment, it's {foo: 2, bar: 1}, but I worry whether this is implementation-specific.)

Answer 1

See ECMAScript section 11.1.5 defining how the ObjectLiteral production is parsed.

In particular:

PropertyNameAndValueList , PropertyName : AssignmentExpression is evaluated as follows:

1. Evaluate PropertyNameAndValueList.

2. Evaluate PropertyName.

3. Evaluate AssignmentExpression.

...

Where (1) is a recursive definition.

This means the leftmost item in an object literal will get evaluated first, and so {foo: 2, bar: 1} is indeed spec-mandated.