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 (inclusive), default 0.

end Optional

End index (exclusive), 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.

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

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

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. 

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

Specifications

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

Browser compatibility

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See also