Array.prototype.fill()

The fill() method fills all the elements of an array from a start index to an end index with a static value.

var numbers = [1, 2, 3]
numbers.fill(1);

// results in [1, 1, 1]

Syntax

arr.fill(value[, start = 0[, end = this.length]])

Parameters

value

Value to fill an array.

start

Optional. Start index.

end

Optional. End index.

Return value

The modified array.

Description

The elements interval to fill is [start, end).

The fill method takes up to three arguments value, start and end. The start and end arguments are optional with default values of 0 and the length of the this object.

If start is negative, it is treated as length+start where length is the length of the array. If end is negative, it is treated as length+end.

The fill function is intentionally generic, it does not require that its this value be an Array object.

The fill method is a mutable method, it will change this object itself, and return it, not just return a copy of it.

Examples

[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, -2);       // [4, 2, 3]
[1, 2, 3].fill(4, NaN, NaN);     // [1, 2, 3]
Array(3).fill(4);                // [4, 4, 4]
[].fill.call({ length: 3 }, 4);  // {0: 4, 1: 4, 2: 4, length: 3}

Polyfill

if (!Array.prototype.fill) {
  Array.prototype.fill = 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 final = relativeEnd < 0 ?
      Math.max(len + relativeEnd, 0) :
      Math.min(relativeEnd, len);

    // Step 12.
    while (k < final) {
      O[k] = value;
      k++;
    }

    // Step 13.
    return O;
  };
}

Specifications

Browser compatibility

[1] Starting with Chrome 36, this was available behind a preference. In chrome://flags, activate the entry “Enable Experimental JavaScript”.

See also

• Array
• TypedArray.prototype.fill()