Split Javascript array elements into chunks at designated indexes

Question

I have an array like so

const arr = [3,6,9,12,18,21,24,27,33,36];

I want the array arr split into chunks at 12, 21 and 33. That is at the index 3, 5, and 8. I want to produce another array chunks looking like this..

const chunks = [[3,6,9,12],[18,21],[24,27,33],[36]];

The solutions I have seen here basically split arrays into 'n' chunks. Basically I want to split at arrays at several (specified) indexes.

I do not mind an underscore.js/lodash solution. Thanks

How is this question a duplicate of split an array into two based on a index in javascript ? The answers there address spliting an array into 2 albeit by index, mine into several parts. — dtechplus
– dtechplus, Commented Oct 31, 2017 at 6:52
It's not exact duplicate, but there is no way to split it in one operation as for example can be done for strings. Each part of the array has to be extracted separately in a loop. — Slai
– Slai, Commented Nov 1, 2017 at 15:02

Sebastian Simon · Accepted Answer · 2022-02-28 21:51:23Z

You could use reduceRight and decide which elements to split at. Since you’re providing the last values of a sub-array rather than the first ones, going from right to left is actually a bit easier, hence I use a reduceRight rather than a reduce.

Split by value

const arr = [3, 6, 9, 12, 18, 21, 24, 27, 33, 36],
  splitValues = [12, 21, 33],
  chunks = arr.reduceRight((result, value) => {
    result[0] = result[0] || [];

    if (splitValues.includes(value)) {
      result.unshift([value]);
    } else {
      result[0].unshift(value);
    }

    return result;
  }, []);

console.log(chunks);

.as-console-wrapper { max-height: 100% !important; top: 0; }

Split by index

const arr = [3, 6, 9, 12, 18, 21, 24, 27, 33, 36],
  splitIndexes = [3, 5, 8],
  chunks = arr.reduceRight((result, value, index) => {
    result[0] = result[0] || [];

    if (splitIndexes.includes(index)) {
      result.unshift([value]);
    } else {
      result[0].unshift(value);
    }

    return result;
  }, []);

console.log(chunks);

.as-console-wrapper { max-height: 100% !important; top: 0; }

CRice · Accepted Answer · 2017-10-31 00:05:54Z

4

const arr = [3,6,9,12,18,21,24,27,33,36];

// Important: this array gets mutated. Make a copy if that's not okay.
const inds = [3,5,8];

const chunked = arr.reduce((p, c, i) => { if (i-1 === inds[0]) { inds.shift(); p.push([]); } p[p.length-1].push(c); return p; }, [[]]);

console.log(chunked)

answered Oct 31, 2017 at 0:05

CRice

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Comments

Slai · Accepted Answer · 2024-07-21 08:21:19Z

4

slice can be used to get part of array:

const arr = [3, 6, 9, 12, 18, 21, 24, 27, 33, 36], indexes = [0, 4, 6, 9];

const chunks = indexes.map((_, i, a) => arr.slice(_, a[i + 1]));

console.log( JSON.stringify(chunks) );

edited Jul 21, 2024 at 8:21

answered Nov 1, 2017 at 18:52

Slai

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Comments

Evan Trimboli · Accepted Answer · 2017-10-31 00:40:27Z

3

Here's an alternative way of doing it that I think is a bit clearer.

function chunkIt(arr, indexes) {
    const ret = [];
    let last = 0;

    indexes.forEach(i => {
        ret.push(arr.slice(last, i + 1));
        last = i + 1;
    });
    if (last < arr.length) {
        ret.push(arr.slice(last));
    }
    return ret;
}

console.log(chunkIt([3,6,9,12,18,21,24,27,33,36], [3,5,8]));

answered Oct 31, 2017 at 0:40

Evan Trimboli

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