Javascript Array copyWithin()

let words = ["apple", "ball", "cat", "dog"];

// copies element from index 0 to index 3 
words.copyWithin(3, 0);

// modifies the original array 
console.log(words);

// Output: 
// [ ''apple'', ''ball'', ''cat'', ''apple'' ]

copyWithin() Syntax

The syntax of the copyWithin() method is:

arr.copyWithin(target, start, end)

Here, arr is an array.

copyWithin() Parameters

The copyWithin() method can take three parameters:

• target - The index position to copy the elements to.
• start (optional) - The index position to start copying elements from. If omitted, it will copy from index 0.
• end (optional) - The index position to stop copying elements from (end element not included). If omitted, it will copy until the last index.

copyWithin() Return Value

• Returns the modified array after copying the elements.

Example 1: Using copyWithin() Method

let numbers = [1, 2, 3, 4, 5];

// copying element located at 4th index to 0th index
numbers.copyWithin(0, 4);

// modifies the original array
console.log(numbers); // [ 5, 6, 3, 4, 5 ]

let letters = ["a", "b", "c", "d"];

// copying element located at 1st index to 2nd index
letters.copyWithin(2, 1);

// modifies the original array 
console.log(letters); // [ 'a', 'b', 'b', 'c' ]

Output

[ 5, 2, 3, 4, 5 ]
[ 'a', 'b', 'b', 'c' ]

In the above example, we have used the copyWithin() method to copy the element from one index to another in the arrays- numbers and letters.

numbers.copyWithin(0, 4) copies element located at start i.e. index 4 to target i.e. index 0 and letters.copyWithin(2, 1) copies element from index 1 to index 2.

Example 2: copyWithin() with Three Parameters

let laptops = ["dell", "hp", "acer", "asus"];

// copying elements from index 2 to 4(excluding 4) to index 0
laptops.copyWithin(0, 2, 4);

// modifies the original array
console.log(laptops); // [ 'acer', 'asus', 'acer', 'asus' ]

Output

[ 'acer', 'asus', 'acer', 'asus' ]

In the above example, we have passed three parameters- target, start, and end in the copyWithin method.

laptops.copyWithin(0, 2, 4) copies elements from index 2 to 4 (not including index 4) to index 0 and modifies the laptops array to [ 'acer', 'asus', 'acer', 'asus' ].

Example 3: copyWithin() with Negative Parameter

let evenNumbers= [2,4,6,8];

// passing negative index value -1 as target index 
evenNumbers.copyWithin(-1);

console.log(evenNumbers);

Output

[ 2, 4, 6, 2 ]

Here we have passed a negative index value -1 as the value for target in the copyWithin() method.

Since the passed argument is negative, the method counts the index backward so the target index is the last index.

evenNumbers.copyWithin(-1) copies the element at index 0(default start value) to last index.