Array.prototype.with()

The with() method of Array instances is the copying version of using the bracket notation to change the value of a given index. It returns a new array with the element at the given index replaced with the given value.

Syntax

arrayInstance.with(index, value)

Parameters

index
- : Zero-based index at which to change the array, converted to an integer.
  - Negative index counts back from the end of the array — if index < 0, index + array.length is used.
  - If the index after normalization is out of bounds, a RangeError is thrown.
value
- : Any value to be assigned to the given index.

Return value

A new array with the element at index replaced with value.

Exceptions

RangeError
- : Thrown if index >= array.length or index < -array.length.

Description

The with() method changes the value of a given index in the array, returning a new array with the element at the given index replaced with the given value. The original array is not modified. This allows you to chain array methods while doing manipulations.

By combining with() with at(), you can both write and read (respectively) an array using negative indices.

The with() method never produces a sparse array. If the source array is sparse, the empty slots will be replaced with undefined in the new array.

The with() method is generic. It only expects the this value to have a length property and integer-keyed properties.

Examples

Creating a new array with a single element changed

const arr = [1, 2, 3, 4, 5];
console.log(arr.with(2, 6)); // [1, 2, 6, 4, 5]
console.log(arr); // [1, 2, 3, 4, 5]

Chaining array methods

With the with() method, you can update a single element in an array and then apply other array methods.

const arr = [1, 2, 3, 4, 5];
console.log(arr.with(2, 6).map((x) => x ** 2)); // [1, 4, 36, 16, 25]

Using with() on sparse arrays

The with() method always creates a dense array.

const arr = [1, , 3, 4, , 6];
console.log(arr.with(0, 2)); // [2, undefined, 3, 4, undefined, 6]

Calling with() on non-array objects

The with() method creates and returns a new array. It reads the length property of this and then accesses each property whose key is a nonnegative integer less than length. As each property of this is accessed, the array element having an index equal to the key of the property is set to the value of the property. Finally, the array value at index is set to value.

const arrayLike = {
  length: 3,
  unrelated: "foo",
  0: 5,
  2: 4,
  3: 3, // ignored by with() since length is 3
};
console.log(Array.prototype.with.call(arrayLike, 0, 1));
// [ 1, undefined, 4 ]