Array.prototype.fill()
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2016.
The fill()
method of Array
instances changes all elements within a range of indices in an array to a static value. It returns the modified array.
Try it
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
-array.length <= start < 0
,start + array.length
is used. - If
start < -array.length
orstart
is omitted,0
is used. - If
start >= array.length
, no index is filled.
- Negative index counts back from the end of the array — if
end
Optional-
Zero-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.length
is used. - If
end < -array.length
,0
is used. - If
end >= array.length
orend
is omitted,array.length
is used, causing all indices until the end to be filled. - If
end
implies a position before or at the position thatstart
implies, nothing is filled.
- Negative index counts back from the end of the array — if
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-keyed 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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