Array.prototype.toSpliced()

start

Zero-based index at which to start changing the array, converted to an integer.

• Negative index counts back from the end of the array — if -array.length <= start < 0, start + array.length is used.
• If start < -array.length or start is omitted, 0 is used.
• If start >= array.length, no element will be deleted, but the method will behave as an adding function, adding as many elements as provided.

deleteCount Optional

An integer indicating the number of elements in the array to remove from start.

If deleteCount is omitted, or if its value is greater than or equal to the number of elements after the position specified by start, then all the elements from start to the end of the array will be deleted. However, if you wish to pass any itemN parameter, you should pass Infinity as deleteCount to delete all elements after start, because an explicit undefined gets converted to 0.

If deleteCount is 0 or negative, no elements are removed. In this case, you should specify at least one new element (see below).

item1, …, itemN Optional

The elements to add to the array, beginning from start.

If you do not specify any elements, toSpliced() will only remove elements from the array.

A new array that consists of all elements before start, item1, item2, …, itemN, and all elements after start + deleteCount.

The toSpliced() method, like splice(), does multiple things at once: it removes the given number of elements from the array, starting at a given index, and then inserts the given elements at the same index. However, it returns a new array instead of modifying the original array. The deleted elements therefore are not returned from this method.

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

You can use toSpliced() to delete, add, and replace elements in an array and create a new array more efficiently than using slice() and concat().

const months = ["Jan", "Mar", "Apr", "May"];

// Inserting an element at index 1
const months2 = months.toSpliced(1, 0, "Feb");
console.log(months2); // ["Jan", "Feb", "Mar", "Apr", "May"]

// Deleting two elements starting from index 2
const months3 = months2.toSpliced(2, 2);
console.log(months3); // ["Jan", "Feb", "May"]

// Replacing one element at index 1 with two new elements
const months4 = months3.toSpliced(1, 1, "Feb", "Mar");
console.log(months4); // ["Jan", "Feb", "Mar", "May"]

// Original array is not modified
console.log(months); // ["Jan", "Mar", "Apr", "May"]

The toSpliced() method always creates a dense array.

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

The toSpliced() method reads the length property of this. It then reads the integer-keyed properties needed and writes them into the new array.

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

• Polyfill of Array.prototype.toSpliced in core-js
• Array.prototype.splice()
• Array.prototype.toReversed()
• Array.prototype.toSorted()
• Array.prototype.with()