How to flatten an array in JavaScript

published: 29 Mar 2022

2 min read

In vanilla JavaScript, you can use the Array.concat() method to flatten a multi-dimensional array. This method works in all modern browsers, and IE6 and above.

Here is an example:

const animals = [
    ['🐍'],
    ['🐢'],
    ['🐝'],
    ['🐉'],
    ['🐋']
];

const flattened = [].concat.apply([], animals);

console.log(flattened);

// ['🐍', '🐢', '🐝', '🐉', '🐋']

The apply() method is a part of concat() function's prototype that calls the function with a given this value, and arguments provided as an array.

Alternatively, you could use the ES6 syntax to make the above example super concise:

const flattened = [].concat(...animals);

console.log(flattened);

// ['🐍', '🐢', '🐝', '🐉', '🐋']

The spread operator (...), we used above, is only available in modern browsers, and doesn't work in IE.

`reduce()` Method

You can also use the Array.reduce() method along with Array.concat() to flatten a multi-dimensional array to a one-dimensional array:

const flowers = [['🍀'], ['🌷'], ['🌹'], ['🌻']];

const flattened = flowers.reduce((flat, val) => flat.concat(val), []);

console.log(flattened);

// ['🍀', '🌷', '🌹', '🌻']

The above approach works in all modern browsers, and IE9 and higher.

`flat()` & `flatMap()` Methods

ES2019 introduced two new methods to Array's prototype, flat() and flatMap(), that can be used to flatten a multi-dimensional array in JavaScript. These methods are fairly new and only works in the latest versions of modern browsers, and Node.js 11 and higher.

The Array.flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth:

const animals = [
    ['🐍'],
    ['🐢'],
    ['🐝'],
    ['🐉'],
    ['🐋']
];

const flattened = animals.flat();

console.log(flattened);

// ['🐍', '🐢', '🐝', '🐉', '🐋']

By default, the depth level is 1. You can pass in an integer to the flat() method to specify how deep a nested array structure should be flattened. Use the Infinity keyword to set it to infinite:

// default depth level
['🍀', ['🌷', ['🌹', '🌻']]].flat();
// ['🍀', '🌷', ['🌹', '🌻']]

// depth level: 2
['🍀', ['🌷', ['🌹', '🌻']]].flat(2);
// ['🍀', '🌷', '🌹', '🌻']

// depth level: infinite
['🍀', ['🌷', ['🌹', '🌻']]].flat(Infinity);
// ['🍀', '🌷', '🌹', '🌻']

The Array.flatMap() is identical to a Array.map() followed by a Array.flat() of depth level 1. This method iterates each element of the array by using a mapping function, and then flattens the results into a new array:

const strings = ['Today is', 'a very', '', 'special day'];

const result = strings.flatMap(str => str.split(' '));

console.log(result);

// ['Today', 'is', 'a', 'very', '', 'special', 'day']

The flat() method is useful when you want to manipulate the nested arrays before they are flattened into a single-dimensional array.

Take a look this article to learn more about JavaScript arrays and how to use them to store multiple values in a single variable.

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How to flatten an array in JavaScript

In vanilla JavaScript, you can use the Array.concat() method to flatten a multi-dimensional array. This method works in all modern browsers, and IE6 and above.

Here is an example:

const animals = [
    ['🐍'],
    ['🐢'],
    ['🐝'],
    ['🐉'],
    ['🐋']
];

const flattened = [].concat.apply([], animals);

console.log(flattened);

// ['🐍', '🐢', '🐝', '🐉', '🐋']

The apply() method is a part of concat() function's prototype that calls the function with a given this value, and arguments provided as an array.

Alternatively, you could use the ES6 syntax to make the above example super concise:

const flattened = [].concat(...animals);

console.log(flattened);

// ['🐍', '🐢', '🐝', '🐉', '🐋']

The spread operator (...), we used above, is only available in modern browsers, and doesn't work in IE.

`reduce()` Method

You can also use the Array.reduce() method along with Array.concat() to flatten a multi-dimensional array to a one-dimensional array:

const flowers = [['🍀'], ['🌷'], ['🌹'], ['🌻']];

const flattened = flowers.reduce((flat, val) => flat.concat(val), []);

console.log(flattened);

// ['🍀', '🌷', '🌹', '🌻']

The above approach works in all modern browsers, and IE9 and higher.

`flat()` & `flatMap()` Methods

The Array.flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth:

const animals = [
    ['🐍'],
    ['🐢'],
    ['🐝'],
    ['🐉'],
    ['🐋']
];

const flattened = animals.flat();

console.log(flattened);

// ['🐍', '🐢', '🐝', '🐉', '🐋']

By default, the depth level is 1. You can pass in an integer to the flat() method to specify how deep a nested array structure should be flattened. Use the Infinity keyword to set it to infinite:

// default depth level
['🍀', ['🌷', ['🌹', '🌻']]].flat();
// ['🍀', '🌷', ['🌹', '🌻']]

// depth level: 2
['🍀', ['🌷', ['🌹', '🌻']]].flat(2);
// ['🍀', '🌷', '🌹', '🌻']

// depth level: infinite
['🍀', ['🌷', ['🌹', '🌻']]].flat(Infinity);
// ['🍀', '🌷', '🌹', '🌻']

const strings = ['Today is', 'a very', '', 'special day'];

const result = strings.flatMap(str => str.split(' '));

console.log(result);

// ['Today', 'is', 'a', 'very', '', 'special', 'day']

The flat() method is useful when you want to manipulate the nested arrays before they are flattened into a single-dimensional array.

Take a look this article to learn more about JavaScript arrays and how to use them to store multiple values in a single variable.