Array.prototype.fill()

fill(value[, start[, end]])

Returns: array · Updated March 13, 2026 · Array Methods

array fill mutation

The fill() method changes all elements in an array to a static value from a start index to an end index. It mutates the original array and returns the modified array.

Syntax

fill(value)
fill(value, start)
fill(value, start, end)

Parameters

Parameter	Type	Default	Description
`value`	`any`	—	The value to fill the array with
`start`	`number`	`0`	Start index (inclusive). Negative counts from the end.
`end`	`number`	`array.length`	End index (exclusive). Negative counts from the end.

Examples

Basic usage

const arr = [1, 2, 3, 4, 5];
arr.fill(0);
console.log(arr);
// [0, 0, 0, 0, 0]

// Fill with a specific value
const nums = [1, 2, 3, 4, 5];
nums.fill(9, 1, 3);
console.log(nums);
// [1, 9, 9, 4, 5]

Using negative indices

const arr = [1, 2, 3, 4, 5];
// Start at second-to-last, fill to end
arr.fill(0, -2);
console.log(arr);
// [1, 2, 3, 0, 0]

// Fill middle portion
arr.fill('x', 1, -1);
console.log(arr);
// [1, 'x', 'x', 'x', 0]

Creating initialized arrays

// Create an array of zeros
const zeros = new Array(5).fill(0);
console.log(zeros);
// [0, 0, 0, 0, 0]

// Create a 2D matrix
const matrix = Array(3).fill(null).map(() => Array(3).fill(0));
console.log(matrix);
// [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

Common Patterns

Initializing a buffer:

const buffer = new ArrayBuffer(16);
const bytes = new Uint8Array(buffer).fill(0xFF);

Resetting an array:

let scores = [10, 20, 30, 40, 50];
scores.fill(0);
console.log(scores);
// [0, 0, 0, 0, 0]