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.
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.)
start
Optional- Start index, default
0
. end
Optional- End index, default
arr.length
.
Return value
The modified array, filled with value
.
Description
- If
start
is negative, it is treated asarray.length + start
. - If
end
is negative, it is treated asarray.length + end
. fill
is intentionally generic: it does not require that itsthis
value be anArray
object.fill
is a mutator method: it will change the array itself and return it, not a copy of it.- If the first parameter is an object, each slot in the array will reference that object.
Polyfill
if (!Array.prototype.fill) {
Object.defineProperty(Array.prototype, 'fill', {
value: function(value) {
// Steps 1-2.
if (this == null) {
throw new TypeError('this is null or not defined');
}
var O = Object(this);
// Steps 3-5.
var len = O.length >>> 0;
// Steps 6-7.
var start = arguments[1];
var relativeStart = start >> 0;
// Step 8.
var k = relativeStart < 0 ?
Math.max(len + relativeStart, 0) :
Math.min(relativeStart, len);
// Steps 9-10.
var end = arguments[2];
var relativeEnd = end === undefined ?
len : end >> 0;
// Step 11.
var finalValue = relativeEnd < 0 ?
Math.max(len + relativeEnd, 0) :
Math.min(relativeEnd, len);
// Step 12.
while (k < finalValue) {
O[k] = value;
k++;
}
// Step 13.
return O;
}
});
}
If you need to support truly obsolete JavaScript engines that don't support Object.defineProperty
, it's best not to polyfill Array.prototype
methods at all, as you can't make them non-enumerable.
Examples
Using fill
[1, 2, 3].fill(4) // [4, 4, 4]
[1, 2, 3].fill(4, 1) // [1, 4, 4]
[1, 2, 3].fill(4, 1, 2) // [1, 4, 3]
[1, 2, 3].fill(4, 1, 1) // [1, 2, 3]
[1, 2, 3].fill(4, 3, 3) // [1, 2, 3]
[1, 2, 3].fill(4, -3, -2) // [4, 2, 3]
[1, 2, 3].fill(4, NaN, NaN) // [1, 2, 3]
[1, 2, 3].fill(4, 3, 5) // [1, 2, 3]
Array(3).fill(4) // [4, 4, 4]
[].fill.call({ length: 3 }, 4) // {0: 4, 1: 4, 2: 4, length: 3}
// A single object, referenced by each slot of the array:
let 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
Specifications
Browser compatibility
BCD tables only load in the browser