JavaScript .copyWithin()

The .copyWithin() method returns a shallow copy of an array where one subarray replaces another part without modifying the original length. However, since it is a mutating method, .copyWithin() will change the array’s contents and create or delete properties if necessary.

Syntax

myArray.copyWithin(target);
myArray.copyWithin(target, start);
myArray.copyWithin(target, start, end);

The following parameters can be applied to the .copyWithin() method:

• target (required): A zero-based index to copy the sequence to.
  • If target is negative, it counts back from myArray.length and target + myArray.length is used.
  • If target is < myArray.length * -1, 0 is used.
  • If target is >= myArray.length, nothing is copied.
  • If, after myArray is normalized, the target is positioned after start, copying only occurs until the end of myArray.length(i.e., .copyWithin() never extends the myArray).
• start (optional): A zero-based index at which to start copying the elements from.
  • If start is negative, it counts back from the end of myArray and start + myArray.length is used.
  • If start < myArray.length * -1 or it is omitted, 0 is used.
  • If start >= myArray.length, nothing is copied.
• end (optional): A zero-based index at which to end copying elements from (non-inclusive).
  • If end is negative, it counts back from the end of myArray and end + myArray.length is used.
  • If end < myArray.length * -1, 0 is used.
  • If end >= myArray.length or it is omitted, myArray.length is used and all elements until the end are copied.
  • If end is positioned before or at start after normalization, nothing is copied.

Sparse Arrays

If the .copyWithin() method is applied to a sparse array that contains empty slots, it will populate them with an undefined value:

console.log([1, , 3].copyWithin(2, 1, 2));
// Output: [1, undefined, undefined]

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Example

The following examples has a few calls to the .copyWithin() method, applying combinations of the available parameters:

// copy elements starting from the second-to-last index
console.log([1, 2, 3, 4, 5].copyWithin(-2));

// copy to index 0 all elements index 3 to the end
console.log([1, 2, 3, 4, 5].copyWithin(0, 3));

// copy to index 0 the element at index 2
console.log([1, 2, 3, 4, 5].copyWithin(0, 2, 3));

// copy to second-to-last index elements starting from index 2 until end of array
console.log([1, 2, 3, 4, 5].copyWithin(-2, -3, -1));

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This will produce the following output:

[ 1, 2, 3, 1, 2 ]
[ 4, 5, 3, 4, 5 ]
[ 3, 2, 3, 4, 5 ]
[ 1, 2, 3, 3, 4 ]

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Codebyte Example

The following example uses the .copyWithin() method to copy the second element at index 1 (‘b’) to the fourth element at index 3:

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