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.)
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jameshfisher
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Why would it be implementation-specific? `pop` mutates the original array. – elclanrs Jul 03 '13 at 00:37
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1@elclanrs, I think what the question is getting at is whether the values of the properties of object `b` will always be evaluated in the order in which they are listed in the code. – go-oleg Jul 03 '13 at 00:42
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Oh, I see... Now that's actually a good question, but while the order of properties in an object is trivial, the order of evaluation is not, so I'd say this is non-issue but I may be wrong... – elclanrs Jul 03 '13 at 00:43
2 Answers
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See ECMAScript section 11.1.5 defining how the ObjectLiteral production is parsed.
In particular:
PropertyNameAndValueList,PropertyName:AssignmentExpressionis evaluated as follows:
Evaluate PropertyNameAndValueList.
Evaluate PropertyName.
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.
