Execution efficiency of forEach function in google V8 engine

[刘玉龙]

I found an interesting thing in V8 engine when I was testing underscore.js library.

For arrays, the 

for (var i = 0; i < len; i++)

implementation can ran 24700 ops(operations per second), while the native function 

Array.prototype.forEach

 only got 14200 ops.

Then I tried IE10, in which the `nativeForEach` can reach a number of 43500.

I checked the source code of V8 engine, but could not find out why this native forEach function is so slow.

Can anybody explain this? Thank you.

[Jakob Kummerow]

Here's what the builtin forEach function does: https://code.google.com/p/v8/source/browse/branches/bleeding_edge/src/array.js#1078

Notably, for every index it checks if the array actually contains that index. This check is mandated by the ECMAScript specification, see http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-262.pdf chapter 15.4.4.18.

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[Sven Panne]

Further note: When your test on IE10 shows that it is almost twice as fast as the plain old for loop, check if your test contains dead code. IE10 is very good at removing this, it easily gives you good benchmark scores without actually doing anything fantastic, because a lot of benchmarks contain lots of dead code. Well-known compiler writer trick for decades... ;-)

[刘玉龙]

OK. I will remember this :)

Anyway, it's not bad if compiler can auto remove the dead code~

在 2013年5月23日星期四UTC+8下午4时57分05秒，Sven Panne写道：

[Kevin Millikin]

Real applications don't normally contain expensive dead code (what programmer would waste their time writing that?).  If you find that your application does contain such code, there is an easy workaround.

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[刘玉龙]

Hey, I have another question.

I noticed that jquery, underscore and some other libraries, don't check if the index is in array. Under what circumstances this lacking of check will cause a bug? Thank you.

在 2013年5月23日星期四UTC+8下午4时51分21秒，Jakob Kummerow写道：

[Jakob Kummerow]

On Fri, May 24, 2013 at 3:53 AM, 刘玉龙 <lyl8...@gmail.com> wrote:

Hey, I have another question.

I noticed that jquery, underscore and some other libraries, don't check if the index is in array. Under what circumstances this lacking of check will cause a bug?

That depends on your definition of "bug"; or, put differently: on your program's expectations how missing elements should be treated when walking over an array. Consider this example:

> var a = [1,,,4]

> a.forEach(function(x) { console.log(x);})

1

4

> for (var i = 0; i < a.length; ++i) console.log(a[i])

1

undefined

undefined

4