Array Chunking

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On several of my side projects, this blog included, I've had to reach for some kind of array chunking code, something that would take a flat array of items and bunch together the items based on some logic frequently based on the number of items I want in each chunk.

While I could probably reach for some _.chunk method, I've always opted to instead write the function locally since it's all of about 10 or 15 lines.

Since I keep on needing this utility, I figured it would be a good exercise to write one and also write about writing one on my blog!

So, without further ado, let's dig in!

Let's start with the problem we're trying to solve, we want to take an array of elements, and would like to return an array of arrays of those items.

Now, we could try and make this super generic to start, but for simplicity sake let's start by trying to create an array chunking function that chunks the array into groups of the same length.

So we have two arguments for our function: array and length, the first is the input array of elements, and the second is the desired chunk length.

function chunk(array, length) {}

Great, let's dive into the implementation, I think I'll want to reach for reduce for the implementation to manage the chunking for us, so let's get that started in our function:

function chunk(array, length) {
  return array.reduce((chunked, item) => {
    // @TODO
  }, []);
}

With the above code, we're returning the result of the reduce call, and we're defaulting the value to an empty array.

Great, now lets get into the really difficult stuff!

To start, the algorithm that we're going to follow will probably look something like:

• Grab the last element of the chunked array
  • This should be an array, or undefined for the first iteration
  • If the last item is undefined, create a new array with the current item and append it onto chunked
• Check the length of this array
  • If the length is equal to the provided length param
    • Create a new array with the current item and append it onto chunked
  • Else, insert the current item into the last element of the chunked array

Alright, let's start with the above steps and see what happens...

function chunk(array, length) {
  return array.reduce((chunked, item) => {
    // grab the last element, note that we're using `.reverse` and rest/spread
    // here to easily grab it and the rest of the chunked array
    let [lastElement, ...rest] = chunked.reverse();
    // If the last element is undefined, then return a new array with the current item
    if (typeof lastElement === 'undefined') {
      return [...rest, [item]];
    }
    // If the last element's length is less than our desired length
    // then append the item to the last element
    if (lastElement.length < length) {
      return [...rest, [...lastElement, item]];
    }
    // finally, return a new array with the current item, since the lastElement's length
    // is our desired `length`
    return [...rest, lastElement, [item]];
  }, []);
}

So there is our solution, let's see how it works! Feel free to copy the below code samples and paste them into devtools or a node repl to test for yourself!

function chunk(array, length) {
  return array.reduce((chunked, item) => {
    let [lastElement, ...rest] = chunked.reverse();
    if (typeof lastElement === 'undefined') {
      return [...rest, [item]];
    }
    if (lastElement.length < length) {
      return [...rest, [...lastElement, item]];
    }
    return [...rest, lastElement, [item]];
  }, []);
}

let arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 11, 22, 33, 44, 55, 66, 77, 88, 99];

chunk(arr, 3);

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