For loop improved with ">>>" operator?

Question

I'm updating mootools from 1.3.2 to 1.4.1. I saw a strange change. From this

for (var i = 0, l = this.length; i < l; i++){....

to this

for (var i = 0, l = this.length >>> 0; i < l; i++){

how the ">>>" operator, used in that way, can improve performance? What do you think about it?

WUT?! But, (5 >>> 0 == 5)... xD – Cipi Nov 24 '11 at 10:24 — Cipi, Nov 24 '11 at 10:24

Rob W · Accepted Answer · 2011-11-24T10:51:06.590

The >>> bitwise operator is bounded between and including 0 and 2^32-1 (4,294,967,295). By using using >>>, the framework ensures that the loop does not execute near-infitite times.

PS. The context of the code:

Array.implement({every: function(fn, bind){
    for (var i = 0, l = this.length >>> 0; i < l; i++){
        if ((i in this) && !fn.call(bind, this[i], i, this)) return false;
    }

Since i is initialised at zero, and incremented by an integer 1, and the length property is always an integer, there are no negative side-effects. Another application of the >>> method is rounding, to convert a decimal number to an integer.

@Antonio Please remember to accept this answer by clicking the tick mark next to it if it solved your question :) — MattiSG, Nov 24 '11 at 18:43

score 2 · Answer 2 · answered Nov 24 '11 at 10:22

2

I guess the reason is some kind of conversion to make sure that the value is always numeric (as opposed to eg. the string '2').

answered Nov 24 '11 at 10:22

erikkallen

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@Cipi not here. `('5' >>> 0) === 5` gives `true` – DiogoDoreto Nov 24 '11 at 10:28
loose typing means that won't matter. '.length' will only be non-numeric on an array-like item (object.length = "foo"); – Dimitar Christoff Nov 25 '11 at 11:19

score 1 · Answer 3 · answered Nov 24 '11 at 14:03

1

Keeto from mootools team shed some light as to 'why' they do this, the answer is, generics and array-likes.

All array methods are supposed to also work as Array.every / Array.prototype.every calls. Which means, you can pass on an object with .length: -1 or similar.

This ensures the length will not be invalid for the loop to fail, I suppose.

Array.prototype.forEach.call({0:1, length: -1});

answered Nov 24 '11 at 14:03

Dimitar Christoff

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*"Keeto from ... and array-likes"* `[citation needed]`. That aside, `-1 >>> 0` equals `4,294,967,295`, which is not equal to the behaviour of a length of -1. The `(i in this)` part of the expression results in the desired effect, but looping 4.3E9 times to achieve the effect of `-1` is extremely inefficient. – Rob W Nov 27 '11 at 11:22
ok, bad example, this, neg values is one thing it won't work on. strings, null, even functions etc --> most will return 0 – Dimitar Christoff Nov 27 '11 at 21:34

spender · Answer 4 · 2011-11-24T10:33:30.213

0

I've seen this "bitshift by zero" technique used as a quick'n'dirty truncate method.

e.g.:

42.8989 >>> 0 == 42

however, this misappropriation of bitshifting might go seriously wrong if you feed in a negative number:

(-42.8989) >>> 0 == 4294967254

edited Nov 24 '11 at 10:33

answered Nov 24 '11 at 10:25

spender

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For loop improved with ">>>" operator?

4 Answers4