The copyWithin() method copies a section of an array to another position within the same array. It modifies the original array in place, which means it does not create a new array but instead updates the existing array with the copied elements. This method is particularly useful for rearranging or duplicating parts of an array.

Syntax

array.copyWithin(target, start, end)

Parameters

• target: The index where the copied elements will be inserted. If target is negative, it is treated as array.length + target, which means it starts from the end of the array.
• start (optional): The index from which to begin copying. If omitted, it defaults to 0. If start is negative, it is treated as array.length + start.
• end (optional): The index at which to stop copying (not including end). If omitted, it defaults to the array's length. If end is negative, it is treated as array.length + end.

Return Value

The copyWithin() method returns the modified array with the copied elements in place. The original array is altered by this method.

Example 1: Basic Usage of copyWithin() Method

const array = [1, 2, 3, 4, 5];
array.copyWithin(0, 3, 4);
console.log(array); // Output: [4, 2, 3, 4, 5]

Explanation:

• array.copyWithin(0, 3, 4) copies the elements from index 3 to 4 (excluding 4) to index 0. As a result, [4] replaces the element at index 0, so the array becomes [4, 2, 3, 4, 5].

Example 2: Copying Elements with Negative Indices

const array = [1, 2, 3, 4, 5];
array.copyWithin(-3, -1);
console.log(array); // Output: [1, 2, 5, 5, 5]

Explanation:

• array.copyWithin(-3, -1) uses negative indices. -3 is treated as 2 (i.e., the third-to-last index), and -1 is treated as 4 (i.e., the last index). The element at index 4 (5) is copied to indices 2, 3, and 4.

Example 3: Copying a Portion of the Array

const array = [1, 2, 3, 4, 5];
array.copyWithin(2, 0, 3);
console.log(array); // Output: [1, 2, 1, 2, 3]

• Explanation:

  array.copyWithin(2, 0, 3) copies the elements from index 0 to 3 (excluding 3) to index 2. This results in [1, 2, 1, 2, 3].

Example 4: Using CopyWithin to Overwrite Elements

const array = [1, 2, 3, 4, 5];
array.copyWithin(1, 3);
console.log(array); // Output: [1, 4, 5, 4, 5]

Explanation:

• array.copyWithin(1, 3) copies elements from index 3 to the end of the array to index 1. Thus, 4 and 5 overwrite 2 and 3, resulting in [1, 4, 5, 4, 5].

Example 5: Handling Edge Cases

const array = [1, 2, 3, 4, 5];
array.copyWithin(1, 5);
console.log(array); // Output: [1, 2, 3, 4, 5]

Explanation:

• array.copyWithin(1, 5) tries to copy elements starting from index 5, which is out of bounds. Since no elements are copied, the array remains unchanged.

Example 6: Copying with Start Greater Than End

const array = [1, 2, 3, 4, 5];
array.copyWithin(1, 3, 1);
console.log(array); // Output: [1, 2, 3, 4, 5]

Explanation:

• array.copyWithin(1, 3, 1) has a start index greater than the end index, which means no elements are copied, and the array remains unchanged.