Order of attribute evaluation in Javascript object literals

Question

I'd like to write something like this (in Javascript):

var obj = { left: left, right: -(left += increment) };

This will only do what I want if I can be sure that attribute values will be evaluated in the same order that I write them. Does the language make such a guarantee anywhere?

Answer 1

You going to expect this to work in browsers?

They don't implement the same language exactly... It wouldn't matter if it was written down somewhere how it should be, the fact is the browsers are different. Decide what browsers you want your code to work on, then test it on those.

If you don't have access to them all, you can make a simple page that graphically displays which result the browser is getting, then submit that page to browsershots.org

Answer 2

From ECMA 3rd Edition specifications:

The source text of an ECMAScript program is first converted into a sequence of input elements, which are either tokens, line terminators, comments, or white space. The source text is scanned from left to right, repeatedly taking the longest possible sequence of characters as the next input element.

In other words, yes, the specification addresses this. This code should always work:

var left = 5, increment = 1;
var obj = { left: left, right: -(left += increment) };
// obj.left  ===  5
// obj.right === -6

This does NOT have to do with the order of attributes as they are stored in an object, but rather, the order in which JavaScript will evaluate your statement: left-to-right, up-to-down.