Array.prototype.fill()

The fill() method changes all elements in an array to a static value, from a start index (default 0) to an end index (default array.length). It returns the modified array.

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Syntax

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

Parameters

value

Value to fill the array with. Note all elements in the array will be this exact value: if value is an object, each slot in the array will reference that object.

start Optional

Zero-based index at which to start filling, converted to an integer.

  • Negative index counts back from the end of the array — if start < 0, start + array.length is used.
  • If start < -array.length or start is omitted, 0 is used.
  • If start >= array.length, no index is filled.
end Optional

Zero-based index at which to end filling, converted to an integer. fill() fills up to but not including end.

  • Negative index counts back from the end of the array — if end < 0, end + array.length is used.
  • If end < -array.length, 0 is used.
  • If end >= array.length or end is omitted, array.length is used, causing all indices until the end to be filled.
  • If end is positioned before or at start after normalization, no index is filled.

Return value

The modified array, filled with value.

Description

The fill() method is a mutating method. It does not alter the length of this, but it will change the content of this.

The fill() method fills empty slots in sparse arrays with value as well.

The fill() method is generic. It only expects the this value to have a length property. Although strings are also array-like, this method is not suitable to be applied on them, as strings are immutable.

Note: Using Array.prototype.fill() on an empty array (length = 0) would not modify it as the array has nothing to be modified. To use Array.prototype.fill() when declaring an array, make sure the array has non-zero length. See example.

Examples

Using fill

console.log([1, 2, 3].fill(4)); // [4, 4, 4]
console.log([1, 2, 3].fill(4, 1)); // [1, 4, 4]
console.log([1, 2, 3].fill(4, 1, 2)); // [1, 4, 3]
console.log([1, 2, 3].fill(4, 1, 1)); // [1, 2, 3]
console.log([1, 2, 3].fill(4, 3, 3)); // [1, 2, 3]
console.log([1, 2, 3].fill(4, -3, -2)); // [4, 2, 3]
console.log([1, 2, 3].fill(4, NaN, NaN)); // [1, 2, 3]
console.log([1, 2, 3].fill(4, 3, 5)); // [1, 2, 3]
console.log(Array(3).fill(4)); // [4, 4, 4]

// A single object, referenced by each slot of the array:
const arr = Array(3).fill({}); // [{}, {}, {}]
arr[0].hi = "hi"; // [{ hi: "hi" }, { hi: "hi" }, { hi: "hi" }]

Using fill() to create a matrix of all 1

This example shows how to create a matrix of all 1, like the ones() function of Octave or MATLAB.

const arr = new Array(3);
for (let i = 0; i < arr.length; i++) {
  arr[i] = new Array(4).fill(1); // Creating an array of size 4 and filled of 1
}
arr[0][0] = 10;
console.log(arr[0][0]); // 10
console.log(arr[1][0]); // 1
console.log(arr[2][0]); // 1

Using fill() to populate an empty array

This example shows how to populate an array, setting all elements to a specific value. The end parameter does not have to be specified. 

const tempGirls = Array(5).fill("girl", 0);

Note that the array was initially a sparse array with no assigned indices. fill() is still able to fill this array.

Calling fill() on non-array objects

The fill() method reads the length property of this and sets the value of each integer property from start to end.

const arrayLike = { length: 2 };
console.log(Array.prototype.fill.call(arrayLike, 1));
// { '0': 1, '1': 1, length: 2 }

Specifications

Specification
ECMAScript Language Specification
# sec-array.prototype.fill

Browser compatibility

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See also