How to Add 2 Arrays Together in JavaScript

To add 2 arrays together in JavaScript, we recommend using the concat() function. The concat() function returns a new array that consists of the two given arrays together.

const array = [1, 2, 3];
const array2 = [4, 5, 6];
const array3 = array.concat(array2); // [1, 2, 3, 4, 5, 6]

array3 === array; // false

push() with spread operator

You can also use the push() method with the spread operator. This approach modifies the array in place instead of creating a new array.

const array = [1, 2, 3];
const array2 = [4, 5, 6];
array.push(...array2); // [1, 2, 3, 4, 5, 6]

Note: Be careful about using this approach with potentially huge arrays. This can cause a stack overflow error if array2 is massive.

const array = [];
const array2 = Array(10_000_000).fill(null);

// RangeError: Maximum call stack size exceeded
array.push(...array2);

Using Immutable Patterns

You can also use the spread operator as an alternative to concat() to create a new array as follows. This approach is syntactically neater, and gives you more flexibility in constructing the new array.

const array = [1, 2, 3];
const array2 = [4, 5, 6];
const array3 = [...array, ...array2]; // [1, 2, 3, 4, 5, 6]

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