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Split Array into `n` number of chunks

Originally published on

Recently, while making my blog, I ran into a problem. I was using headless chrome to take snapshot of the URLs provided on the Works page, and it was turning out to be quite slow, even on Cloud Servers, which have top-notch internet and processing speed.

I was passing it a URL at a time, it would go to it, take a snapshot, then move to the next URL, and so on. It was a very linear process.

But those servers being so powerful, they could've snapshot 5 pages at once at the speed it takes for 1 page. So I decided to split the work up in chunks. I broke the urls up in chunks of 5. Chrome would snapshot those 5 simultaneously, and when it was done with all, it would move on to the next 5, and so on. The times reduced to a third of the time.

Consider this array list. For our purposes, let's just deal simply with numbers rather than URLs.

const list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];
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Now I want to split it into chunks of 5, like this,

const splitList = [
  [1, 2, 3, 4, 5],
  [6, 7, 8, 9, 10],
  [11, 12],
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Or chunks of 4:

const SplitList = [
  [1, 2, 3, 4],
  [5, 6, 7, 8],
  [9, 10, 11, 12],
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You get the idea.


The code for splitting an array in its simplest form would be:

 * @param {Array} arr The array to be broken into chunks
 * @param {number} n The number of items in each chunk
function chunks(arr, n) {
  let items = [...arr];
  return new Array(Math.ceil(items.length / n))
    .map(() => items.splice(0, n));
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Let's break it down piece by piece:

  • We're returning an Array with the number of items defined by Math.ceil(items.length / n). if items.length is 12, and n is 5, we would get the devision as 2.4 . More than 2, less than 3. 2 items(11, 12) will be leftover, and will need to be accomodated in 3rd array, so Math.ceil the division.

  • .fill() simply fills the array up with undefined. This is to get an array with the required size, and we can modify those values later. The example above would return [undefined, undefined, undefined].

  • .map() We're traversing over every value in the array of undefined and replacing it with a chunk of the items array. splice removes a chunk from the given array and returns as the value for the .map

  • Lastly, notice we're cloning arr into items. This is required to not mutate the original value.


Let's see how splice works

const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9];

const spliced = arr.splice(0, 3);

// let's print both the original array and spliced array
console.log(spliced); // Output: [1, 2, 3]

console.log(arr); // [4, 5, 6, 7, 8, 9]
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splice mutated the original array.

That's what would happen in the chunks function.

const arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12];

const chunked = chunks(arr, 5);

console.log(arr); // Output: []
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chunks would remove all the values from the original array. However due to cloning the passed array and working on it, we avoid this problem.


Go chunk it up 😉 😜 🤣

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