Flatten an Array One Level Deep in Javascript

In this article, we will see how to flatten an array using native javascript and also a glimpse of lodash(an open-source JavaScript utility library) _.flatten method.

During development, the developer needs to go through loops to prepare a single dimensional array out of a multi-dimensional array or flatten an array in JavaScript.

Array Flatten One Level Deep

Using reduce method

Now using reduce method

let nestedArray = [[12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]];

// Native
const flatten = nestedArray.reduce((a, b) => a.concat(b), []);
console.log(flatten);
// Output => [12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]];

Using concat method

Now using concat method

let nestedArray = [[12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]];

const flatten = [].concat(...nestedArray);
console.log(flatten);
// Output => [12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]

Using flat method introduced in ES2019

Now using flat method introduced in ES2019

let nestedArray = [[12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]];

// Native(ES2019)
const flatten = nestedArray.flat();
console.log(flatten);
// Output => [12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]

Using flatMap method

Now using flatMap method

let nestedArray = [[12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]];

const flatten = nestedArray.flatMap((number) => number);
console.log(flatten);
// Output => [12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]

Array Flatten Using Lodash/Underscore library

Now using _.flatten method from Lodash/Underscore utility library.

let nestedArray = [[12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]];
// Underscore/Lodash
_.flatten(nestedArray);
console.log(flatten);
// Output => [12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]

Infinitely Nested Arrays using flat method introduced in ES2019

Suppose you want flatten an array infinitely deep. We just need to pass Infinity within flat method introduced in ES2019.

const infiniteDeep = [[12, [2, 4], 124, 7, 10, 8, [15, [6, 7]]]];

console.log(infiniteDeep.flat(Infinity));
// [12, 2, 4, 124, 7, 10, 8, 15, 6, 7]

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