Array.prototype.copyWithin()

The copyWithin() method shallow copies part of an array to another location in the same array and returns it without modifying its length.

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.copyWithin(target[, start[, end]])

Parameters

target

Zero-based index at which to copy the sequence to. If negative, target will be counted from the end.

If target is at or greater than arr.length, nothing will be copied. If target is positioned after start, the copied sequence will be trimmed to fit arr.length.

start Optional

Zero-based index at which to start copying elements from. If negative, start will be counted from the end.

If start is omitted, copyWithin will copy from index 0.

end Optional

Zero-based index at which to end copying elements from. copyWithin copies up to but not including end. If negative, end will be counted from the end.

If end is omitted, copyWithin will copy until the last index (default to arr.length).

Return value

The modified array.

Description

The copyWithin works like C and C++'s memmove, and is a high-performance method to shift the data of an Array. This especially applies to the TypedArray method of the same name. The sequence is copied and pasted as one operation; pasted sequence will have the copied values even when the copy and paste region overlap.

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

The copyWithin method is a mutable method. It does not alter the length of this, but it will change its content and create new properties, if necessary.

Polyfill

if (!Array.prototype.copyWithin) {
  Object.defineProperty(Array.prototype, 'copyWithin', {
    value: function(target, start/*, end*/) {
    // 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 6-8.
    var relativeTarget = target >> 0;

    var to = relativeTarget < 0 ?
      Math.max(len + relativeTarget, 0) :
      Math.min(relativeTarget, len);

    // Steps 9-11.
    var relativeStart = start >> 0;

    var from = relativeStart < 0 ?
      Math.max(len + relativeStart, 0) :
      Math.min(relativeStart, len);

    // Steps 12-14.
    var end = arguments[2];
    var relativeEnd = end === undefined ? len : end >> 0;

    var final = relativeEnd < 0 ?
      Math.max(len + relativeEnd, 0) :
      Math.min(relativeEnd, len);

    // Step 15.
    var count = Math.min(final - from, len - to);

    // Steps 16-17.
    var direction = 1;

    if (from < to && to < (from + count)) {
      direction = -1;
      from += count - 1;
      to += count - 1;
    }

    // Step 18.
    while (count > 0) {
      if (from in O) {
        O[to] = O[from];
      } else {
        delete O[to];
      }

      from += direction;
      to += direction;
      count--;
    }

    // Step 19.
    return O;
  },
  configurable: true,
  writable: true
  });
}

Examples

Using copyWithin

[1, 2, 3, 4, 5].copyWithin(-2)
// [1, 2, 3, 1, 2]

[1, 2, 3, 4, 5].copyWithin(0, 3)
// [4, 5, 3, 4, 5]

[1, 2, 3, 4, 5].copyWithin(0, 3, 4)
// [4, 2, 3, 4, 5]

[1, 2, 3, 4, 5].copyWithin(-2, -3, -1)
// [1, 2, 3, 3, 4]

[].copyWithin.call({length: 5, 3: 1}, 0, 3)
// {0: 1, 3: 1, length: 5}

// ES2015 Typed Arrays are subclasses of Array
var i32a = new Int32Array([1, 2, 3, 4, 5])

i32a.copyWithin(0, 2)
// Int32Array [3, 4, 5, 4, 5]

// On platforms that are not yet ES2015 compliant:
[].copyWithin.call(new Int32Array([1, 2, 3, 4, 5]), 0, 3, 4);
// Int32Array [4, 2, 3, 4, 5]

Specifications

Browser compatibility

See also

• Array