JavaScript - string versus integer key

Question

I was running performance benchmarks for jQuery (don't ask) and discovered something interesting. For some reason it seems that this[0] = element is quite slow compared to this.foo = element. Here is the obligatory jsPerf case.

Can anybody explain why there is such a performance hit? Is there any way to improve the performance apart from the obvious "use a string key"?

could you give us a sense of the difference in performance? (ie, 30% slower vs 100x slower) — BostonJohn
– BostonJohn, Commented Oct 12, 2012 at 17:07
Some other test jsperf.com/test-string-and-index . I guess the performance hit has to do something with array. — nhahtdh
– nhahtdh, Commented Oct 12, 2012 at 17:11
Speed is irrelevant if you're using a property that doesn't qualify as a valid identifier. In those cases, you need to use [] irrespective of performance differences. — I Hate Lazy
– I Hate Lazy, Commented Oct 12, 2012 at 17:14
Also, be sure to test in your actual environment instead of just jsPerf. I don't trust it. — I Hate Lazy
– I Hate Lazy, Commented Oct 12, 2012 at 17:18
Property that doesn't qualify as valid identifier will cause very bad performance hit on Chrome. Fun thing is there is almost no difference on IE. — nhahtdh
– nhahtdh, Commented Oct 12, 2012 at 17:48

LetterEh · Accepted Answer · 2012-10-12 17:20:04Z

2

The [0] has to be found to be incompatible, cast to a string ("0"), and then used as a string lookup ["0"].

I don't know that it explains 100% of what's going on, but that's the universal part, anyway.
Most of the rest would be under-the-hood optimizations, on a per-engine basis.

This is also not an array, nor an array-lookup.
It's just bracket-notation on an object.

answered Oct 12, 2012 at 17:20

LetterEh

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2 Comments

pimvdb Over a year ago

But on an array [0] is faster than ['0']: jsperf.com/string-integer-property/5.

LetterEh Over a year ago

That's because it's an array. Arrays use integers and objects use strings.

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