Adding items to array

JavaScript performance comparison

Revision 6 of this test case created by Kyle Simpson and last updated

Info

Various of ways to add items to an array

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Testing in unknown unknown
Test Ops/sec
Native push
 
var a = [];
for (var i = 0; i < 2000; i++) {
 a.push(i);
};
 pending…
Native unshift
 
var a = [];
for (var i = 0; i < 2000; i++) {
 a.unshift(i);
};
 pending…
length
 
var a = [];
for (var i = 0; i < 2000; i++) {
 a[a.length] = i;
};
 pending…
Cached length
 
var a = [];
for (var length = 0; length < 2000; length++) {
 a[length] = length;
};
 pending…
Concat
 
var a = [];
for (var i = 0; i < 2000; i++) {
 a.concat([i]);
};
 pending…
alternate unshift
 
var a = [];
for (var i = 0; i < 2000; i++) {
 a = [i].concat(a)
};
 pending…
Seeded push 1
 
var a = new Array(2000);
for (var i = 0; i < 2000; i++) {
 a.push(i);
};
 pending…
Seeded push 2
 
var a = [];
a.length = 2000;
for (var i = 0; i < 2000; i++) {
 a.push(i);
};
 pending…
Seeded unshift 1
 
var a = new Array(2000);
for (var i = 0; i < 2000; i++) {
 a.unshift(i);
};
 pending…
Seeded unshift 1
 
var a = [];
a.length = 2000;
for (var i = 0; i < 2000; i++) {
 a.unshift(i);
};
 pending…

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2 comments

Sam Foster commented : ∞

The a[a.length] = i results are counter-intuitive. You’d think accessing length in each iteration would be slower. I could’ve sworn I’ve done this test before and got the opposite result, with push(i) being the fastest. Very interesting.

Jonas Fischer commented : ∞

I think that a[a.length] = can be faster in some cases because it do not lookup the push function.

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