Web/JavaScript/Reference/Global objects/Array/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.

The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Syntax

arr.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 as array.length + start.
If end is negative, it is treated as array.length + end.
fill is intentionally generic: it does not require that its this value be an Array 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

let arr = new Array(3);
arr.fill(new Array(4)); // Each value of the array arr is a reference to newly created array of size 4
arr[0][0] = 10;
console.log(arr[0][0]); // 10
console.log(arr[1][0]); // 10
console.log(arr[2][0]); // 10

Specifications

Specification

ECMAScript (ECMA-262)The definition of 'Array.prototype.fill' in that specification.

Browser compatibility

Update compatibility data on GitHub

	Desktop						Mobile						Server
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet	Node.js
`fill`	Chrome Full support 45	Edge Full support 12	Firefox Full support 31	IE No support No	Opera Full support 32	Safari Full support 8	WebView Android Full support 45	Chrome Android Full support 45	Firefox Android Full support 31	Opera Android Full support 32	Safari iOS Full support 8	Samsung Internet Android Full support 5.0	nodejs Full support 4.0.0 Full support 4.0.0 Full support 0.12 Disabled' Disabled' From version 0.12: this feature is behind the `--harmony` runtime flag.

Legend

Full support: Full support
No support: No support
User must explicitly enable this feature.': User must explicitly enable this feature.