If you ever implemented a dynamic array in low level languages like C++ you allocate a chunk of memory for several elements at once, when there's no more space for new elements you allocate a new chunk. I guess this is valid and for Javascript arrays. Allocation of a memory on the heap is an expensive operation.
Let's try in Chrome's console (several times, the output is the most frequent):
const start = performance.memory.usedJSHeapSize;
const arr = new Array(1);
setTimeout(() => {
const diff = performance.memory.usedJSHeapSize - start;
console.log('heap allocated', diff);
}, 100);
> heap allocated 1688
const start = performance.memory.usedJSHeapSize;
const arr = new Array(1_000_000);
setTimeout(() => {
const diff = performance.memory.usedJSHeapSize - start;
console.log('heap allocated', diff);
}, 100);
> heap allocated 4001684
As we see with 1_000_000 array length we have 3.8Mb allocated.
Of course the reason behind the optimization could be different or composite, I'm not a developer of any current JS engines, but it seems that changing the length of an array is generally an expensive operation.
Regardless of the actual JS implementation if we follow the rule:
When the final size of an array is known during creation of the array - allocate it:
const arr = Array(knownSize);
This immediately boosts your Javascript speed:
let $length = 10;
const arr = Array.from({length: $length}, (_, i) => i);
// @benchmark non pre-allocated
{
const result = [];
for(let i = 0; i < arr.length; i++){
result.push(arr[i]);
}
result;
}
// @benchmark pre-allocated
{
const result = Array(arr.length);
for(let i = 0; i < arr.length; i++){
result[i] = arr[i];
}
result;
}
// @benchmark Array#map
arr.map(i => i);
` Chrome/125
-----------------------------------------------------------------------------------------------
> n=10 | n=100 | n=1000 | n=10000
pre-allocated ■ 1.00x x10m 85 | ■ 1.00x x10m 669 | ■ 1.00x x1m 474 | ■ 1.00x x100k 560
Array#map 1.69x x10m 144 | 1.79x x1m 120 | 2.15x x100k 102 | 3.20x x10k 179
non pre-allocated 3.22x x10m 274 | 3.57x x1m 239 | 4.11x x100k 195 | 3.66x x10k 205
----------------------------------------------------------------------------------------------- `
` Firefox/126
---------------------------------------------------------------------------------------------
> n=10 | n=100 | n=1000 | n=10000
pre-allocated ■ 1.00x x10m 227 | ■ 1.00x x1m 162 | ■ 1.00x x100k 192 | ■ 1.00x x10k 208
non pre-allocated 1.44x x10m 327 | 1.45x x1m 235 | 1.16x x100k 222 | 1.53x x10k 319
Array#map 1.46x x10m 332 | 1.06x x1m 172 | 1.73x x100k 333 | 2.17x x10k 452
--------------------------------------------------------------------------------------------- `
Keep in mind though that you create a sparse array with Array(size) so some methods like map(), forEach() wouldn't work as expected, but the iterators works fine like [...arr].
Top comments (8)
Amazing observation, thanks for sharing!
Would it mean that
arr.reduce((a, c, i) => {a[i] = c; return a }, Array(arr.length))actually beats a plainarr.map(i => i);in terms of performance due to the mere array pre-allocation? On paper it looks like it should.Beats in Chrome, but it shouldn't since the array size in
Array#map()is known and should be pre-allocated too, but I found that many native array methods are slower than custom implementations, probably a topic for some future post with examples.Open in the playground
That's an interesting topic that I didn't see being commented very often. I guess many JS developers don't know about memory allocation, and I really don't blame them, I think that it's just uncommon to deal with issues like that nowadays and if you don't even know that this thing exists, how can you address it? Good job on the benchmark measuring as a proof.
What is the tool you're using for benchmark?
silentmantra.github.io/benchmark
Press
helpto see a simple default template.Currently refactoring it to make open-source.
I don't know how to read those benchmark results, but I bring the MDN documentation on the Array constructor. It says, for the parameter of the constructor in use here:
It basically says that preallocation is not a thing in JavaScript. The constructor merely creates the array object and sets its
lengthproperty value to the value passed to the constructor. There is no memory preallocation as in "not slots with actual undefined values".Benchmarks clearly show that if you create an array with a predefined size and doesn't change its length while filling it, it's faster than working with dynamic length. Regarding sparse arrays - it doesn't mean it's not allocated, it COULD mean that allocated values are
undefined. Regardless of ACTUAL implementation, so called "pre-allocation" in my example shows stable speed benefits over long time (probably years) in Chrome and Firefox.That's it, the docs clearly say it is not
undefinedvalues. It makes it clear there is no pre-allocation done. That's the spec, though. In reality, maybe browsers do pre-allocate. Who knows, right? (I don't know, BTW). I was, however, compelled to point out that the documentation is clear about it: Pre-allocation is not expected at all.performance.memoryand benchmarking tell otherwise 🤷♂️