JavaScript Append to Array

Oct 20, 2020

JavaScript arrays have a push() method that add one or more elements to the end of an array.

const arr = ['a', 'b', 'c'];

arr; // ['a', 'b', 'c', 'd']

arr.push('e', 'f');
arr; // ['a', 'b', 'c', 'd', 'e', 'f']

Append to the Beginning

The push() function adds a new element to the end of the array. To add elements to the beginning, you should use the unshift() method.

const arr = ['d', 'e', 'f'];

arr; // ['c', 'd', 'e', 'f']

arr.unshift('a', 'b');
arr; // ['a', 'b', 'c', 'd', 'e', 'f']

Append to Middle

To append an element to somewhere other than the beginning or end of the array, use the splice() method.

const arr = ['a', 'b', 'd'];

let start = 2;
let deleteCount = 0;
arr.splice(start, deleteCount, 'c');

arr; // ['a', 'b', 'c', 'd'];

Immutable Methods

Some frontend apps (often apps built with React) rely on immutability for faster comparisons of large objects. The push(), unshift(), and splice() methods modify the array in place, so you can't use them in apps where immutability is a concern.

To add elements to the end or beginning of the array, you can use the concat() method:

let arr = ['c'];

arr = arr.concat(['d', 'e']);
arr; // ['c', 'd', 'e']

// You can also use `concat()` to add to the beginning of
// the array, just make sure you call `concat()` on an array
// containing the elements you want to add to the beginning.
arr = ['a', 'b'].concat(arr);
arr; // ['a', 'b', 'c', 'd', 'e']

Another common pattern is using the spread operator.

let arr = ['c'];

// Append to the end:
arr = [...arr, 'd', 'e'];
arr; // ['c', 'd', 'e']

// Append to the beginning:
arr = ['a', 'b', ...arr];
arr; // ['a', 'b', 'c', 'd', 'e']

arr = ['c'];
// Append to the middle:
arr = ['a', 'b', ...arr, 'd', 'e'];
arr; // ['a', 'b', 'c', 'd', 'e']

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