Array.prototype.fill()

The fill() method fills (modifies) all the elements of an array from a start index (default zero) to an end index (default array length) with a static value. It returns the modified array.

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Syntax

arr.fill(value[, start[, end]])

Parameters

value

Value to fill an array.

start Optional

Start index, defaults to 0.

end Optional

End index, defaults to arr.length.

Return value

The modified array.

Description

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 array.

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.

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

fill is a mutator method, it will change array itself, and return it, not just return a copy of it.

When the first parameter given to fill is an object, it will copy the reference and fill the array with references to that object.

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, 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}

// Objects by reference.
var arr = Array(3).fill({}) // [{}, {}, {}];
arr[0].hi = "hi"; // [{ hi: "hi" }, { hi: "hi" }, { hi: "hi" }]

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 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;
    }
  });
}

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.

Specifications

Browser compatibility

See also

• Array
• TypedArray.prototype.fill()