Created
March 19, 2014 02:34
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Think that properties are always enumerated in insertion order in JavaScript? Think again.
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| # The following is in Node, but Firefox has the same behavior | |
| $ node --version | |
| v0.10.26 | |
| tinker:~ greg$ node | |
| > var x = {"z": 0, "2": 1, "0": 2, "a": 3, "-3": 4, "b": 5, "-5": 6, "1": 7} | |
| undefined | |
| > Object.keys(x) | |
| [ '0', | |
| '1', | |
| '2', | |
| 'z', | |
| 'a', | |
| '-3', | |
| 'b', | |
| '-5' ] | |
| > for (key in x) { console.log(key) } | |
| 0 | |
| 1 | |
| 2 | |
| z | |
| a | |
| -3 | |
| b | |
| -5 | |
| undefined |
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This caught me by surprise today since node.js and most browsers typically maintain insertion order. Apparently when the keys are strings representing non-negative integers, they are moved to the front and put in order.