V8 Optimization Killers

Re:V8 Optimization Killers by phpnode on Wednesday June 25, 2014 @09:05AM (#7943618)

`for ... in` is a relatively slow construct anyway, the faster option (a lot more verbose) is:

    var keys = Object.keys(obj),
        length = keys.length,
        key, i;
    for (i = 0; i < length; i++) {
      key = keys[i];
    }

While this is not exactly the same as for..in, it usually behaves how you'd expect and is significantly faster for a couple of reasons:

1. In a for..in loop the engine must keep track of the keys already iterated over, whereas in the fast version we can simply increment a counter and do a fast array lookup.

2. It's possible to add properties to the object that you're iterating within the body of the for..in statement, and these will be iterated too. Doing such a thing is obviously very rare but it's the kind of edge case the JS engine must keep track of.

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Re:V8 Optimization Killers by kipple on Wednesday June 25, 2014 @12:26PM (#7944969)

Is a plain for loop with a counter significantly faster than a forEach? With Object.keys I've been enjoying the simplicity of statements like

  Object.keys(obj).forEach(function (key) {
    console.log(obj[key]);
  });

but does that hamstring my performance?

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Re:V8 Optimization Killers by bzbarsky on Wednesday June 25, 2014 @01:08PM (#7945291)

Your mileage will vary depending on the browser. If you really care (as in profiling identified this as a hotspot) you should measure the alternatives you're considering.

But note that in your specific case chances are the cost of a bunch of console.log() calls completely swamps the cost of either a for loop or a forEach call. console.log() has _incredibly_ complicated behavior.

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Re:V8 Optimization Killers by ekmartin on Thursday June 26, 2014 @02:27AM (#7948367)

A native counter for-loop will always be faster: http://jsperf.com/angular-foreach-vs-native-for-loop/13

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Re:V8 Optimization Killers by Alphasite_ on Wednesday June 25, 2014 @11:10AM (#7944424)

Is that (#2) defined behaviour, or is it undefined? Because iirc, other languages disallow it for similar reasons.

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Re:V8 Optimization Killers by gsnedders on Wednesday June 25, 2014 @05:09PM (#7946773)

Adding properties is undefined in ES5 and I believe defined in ES6.

Deleting properties is guaranteed to not break iteration and deleting an as-of-yet unvisited property causes it to never be visited.

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Re:V8 Optimization Killers by general_failure on Wednesday June 25, 2014 @11:31AM (#7944563)

Object.keys() does not need to malloc() ?

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Re:V8 Optimization Killers by kevingadd on Wednesday June 25, 2014 @08:03PM (#7947458)

Keys is an alloc. That is not 'faster'.

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