Array.prototype.fill()
BaselineWidely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.
Thefill() method ofArray instances changes all elements within a range of indices in an array to a static value. It returns the modified array.
Try it
const array1 = [1, 2, 3, 4];// Fill with 0 from position 2 until position 4console.log(array1.fill(0, 2, 4));// Expected output: Array [1, 2, 0, 0]// Fill with 5 from position 1console.log(array1.fill(5, 1));// Expected output: Array [1, 5, 5, 5]console.log(array1.fill(6));// Expected output: Array [6, 6, 6, 6]Syntax
fill(value)fill(value, start)fill(value, start, end)Parameters
valueValue to fill the array with. Note all elements in the array will be this exact value: if
valueis an object, each slot in the array will reference that object.startOptionalZero-based index at which to start filling,converted to an integer.
- Negative index counts back from the end of the array — if
-array.length <= start < 0,start + array.lengthis used. - If
start < -array.lengthorstartis omitted,0is used. - If
start >= array.length, no index is filled.
- Negative index counts back from the end of the array — if
endOptionalZero-based index at which to end filling,converted to an integer.
fill()fills up to but not includingend.- Negative index counts back from the end of the array — if
-array.length <= end < 0,end + array.lengthis used. - If
end < -array.length,0is used. - If
end >= array.lengthorendis omitted orundefined,array.lengthis used, causing all indices until the end to be filled. - If
endimplies a position before or at the position thatstartimplies, nothing is filled.
- Negative index counts back from the end of the array — if
Return value
The modified array, filled withvalue.
Description
Thefill() method is amutating method. It does not alter the length ofthis, but it will change the content ofthis.
Thefill() method fills empty slots insparse arrays withvalue as well.
Thefill() method isgeneric. It only expects thethis value to have alength property. Although strings are also array-like, this method is not suitable to be applied on them, as strings are immutable.
Note:UsingArray.prototype.fill() on an empty array (length = 0) would not modify it as the array has nothing to be modified.To useArray.prototype.fill() when declaring an array, make sure the array has non-zerolength.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 theones() 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]); // 10console.log(arr[1][0]); // 1console.log(arr[2][0]); // 1Using fill() to populate an empty array
This example shows how to populate an array, setting all elements to a specific value.Theend parameter does not have to be specified.
const tempGirls = Array(5).fill("girl", 0);Note that the array was initially asparse array with no assigned indices.fill() is still able to fill this array.
Calling fill() on non-array objects
Thefill() method reads thelength property ofthis and sets the value of each integer-keyed property fromstart toend.
const arrayLike = { length: 2 };console.log(Array.prototype.fill.call(arrayLike, 1));// { '0': 1, '1': 1, length: 2 }Specifications
| Specification |
|---|
| ECMAScript® 2026 Language Specification # sec-array.prototype.fill |