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 thanarr.length
, nothing will be copied. Iftarget
is positioned afterstart
, the copied sequence will be trimmed to fitarr.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 index0
. end
Optional- Zero-based index at which to end copying elements from.
copyWithin
copies up to but not includingend
. If negative,end
will be counted from the end. - If
end
is omitted,copyWithin
will copy until the last index (default toarr.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
Specification |
---|
ECMAScript (ECMA-262) The definition of 'Array.prototype.copyWithin' in that specification. |
Browser compatibility
BCD tables only load in the browser