Flatten an array in Javascript

Flattening an array means making it one-dimensional. The easiest way to flatten an array is via Array.prototype.flat.

// multi-dimensional array
const multiDimensional = [1, 2, [3, 4]];

// flat, one-dimensional array
const oneDimensional = [1, 2, 3, 4];

For many years, doing this in javascript required the use of concat, spread, or reduce. Not anymore.

Flatten with Array.prototype.flat

This method does exactly what you’d expect:

const multiDimensional = [1, 2, [3, 4]];

const oneDimensional = multiDimensional.flat();
// [1, 2, 3, 4]

By default, flat only flattens one level deep:

const multiDimensional = [1, [2, [3, 4]]];

// [3, 4] is nested two levels deep
multiDimensional.flat();
// [1, 2, [3, 4]]

But flat accepts an argument specifying depth:

const multiDimensional = [1, [2, [3, 4]]];
multiDimensional.flat(2);
// [1, 2, 3, 4]

That’s it!

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