for vs forEach

JavaScript performance comparison

Revision 68 of this test case created

Info

Is it faster to use the native forEach or just loop with for?

Inspired by Adrian Sutton's tests at: http://www.symphonious.net/2010/10/09/javascript-performance-for-vs-foreach/

This one adds random floating point numbers to see if the loop overhead is significant at all in the face of standard work.

Preparation code

<script src="//ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js">
</script>
<script>
Benchmark.prototype.setup = function() {
    var i,
        value,
        length,
        values = [],
        sum = 0,
        context = values;
   
   
    for (i = 0; i < 10000; i++) {
        values[i] = Math.random();
    }
   
    function add(val) {
        sum += val;
    }
};

Benchmark.prototype.teardown = function() {
    i = 0;
    value = 0;
    length = 0;
    values = [];
    sum = 0;
};
</script>

Test runner

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Testing in unknown unknown
Test Ops/sec
forEach
values.forEach(add);
pending…
for loop, simple
for (i = 0, k = values.length; i < k; ++i) {
    sum += values[i];
}
pending…
for loop, cached length
for (i = 0, k = values.length; i < k; i++) {
    sum += values[i];
}
pending…
for loop, reverse
for (i = values.length - 1; i >= 0; i--) {
    sum += values[i];
}
pending…
for loop, cached length, callback
for (i = values.length - 1; i >= 0; --i) {
    sum += values[i];
}
pending…
for loop, cached length, callback.call
length = values.length;
for (i = 0; i < length; i++) {
    add.call(context, values[i], i, values);
}
pending…
$.each
$.each(values, function(key, value) {
    sum += value;
});
pending…
for ... in
for (i in values) {
    sum += values[i];
}
pending…
for loop, reverse, decrement condition
for (i = values.length; i--;) {
    sum += values[i];
}
pending…
for loop, reverse, pre-decrement
for (i = values.length - 1; i >= 0; --i) {
    sum += values[i];
}
pending…
for loop, assignment condition
for (i = 0; (value = values[i]) !== undefined; i++) {
    sum += value;
}
pending…
for loop, assignment condition, reversed
for (i = values.length - 1; (value = values[i]) !== undefined; i--) {
    sum += value;
}
pending…
for loop, assignment condition, callback
for (i = 0; (value = values[i]) !== undefined; i++) {
    add(value, i, values);
}
pending…
for loop, assignment condition, callback.call
for (i = 0; (value = values[i]) !== undefined; i++) {
    add.call(context, value, i, values);
}
pending…

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1 comment

kybernetikos commented :

It seems to me that it's a little unfair that the cases that use 'add' are incrementing a variable from a scope that is closed over, whereas the simple for loop cases are incrementing a variable from the current scope.

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