What does splice() do?

JavaScriptJunior~1 min read

It can remove, add and replace elements right in the original array.

Syntax

javascript

array.splice(start, deleteCount, item1, item2, ...);

Parameter	What it does
`start`	The index at which changes start
`deleteCount`	How many elements to remove
`item1, item2, ...`	Elements to insert in place of the removed ones (optional)

1. Removing elements

javascript

const arr = ["a", "b", "c", "d"];
const removed = arr.splice(1, 2);

console.log(arr);     // ["a", "d"]
console.log(removed); // ["b", "c"]

From position 1, remove 2 elements.
Returns an array of the removed elements.
The original array is changed.

2. Adding elements

javascript

const arr = ["a", "d"];
arr.splice(1, 0, "b", "c");

console.log(arr); // ["a", "b", "c", "d"]

deleteCount = 0 means nothing is removed.
We add "b" and "c" at position 1.
Everything that came after shifts to the right.

3. Replacing elements

javascript

const arr = ["a", "b", "c"];
arr.splice(1, 1, "x", "y");

console.log(arr); // ["a", "x", "y", "c"]

Starting at index 1, remove 1 element ("b")
and insert "x", "y" in its place.
You can think of this as removal + insertion at the same time.

4. Using negative indexes

javascript

const arr = [1, 2, 3, 4];
arr.splice(-2, 1); // remove one element starting from the end

console.log(arr); // [1, 2, 4]

-2 means "the second element from the end".

5. If you remove more than exists

javascript

const arr = [1, 2];
arr.splice(1, 10);
console.log(arr); // [1]

The method does not throw an error, it just removes everything there is starting from position 1.

In short:

What it does	Example	Result
Removes	`arr.splice(1, 2)`	Removes 2 elements starting at index 1
Adds	`arr.splice(2, 0, "x")`	Inserts "x" at position 2
Replaces	`arr.splice(1, 1, "x")`	Changes 1 element to "x"
Returns	`["removed elements"]`	array of removed values
Mutates the original array	Yes

An "all at once" example

javascript

const arr = [1, 2, 3, 4, 5];

// remove 2 elements starting at index 1 and insert new ones
const removed = arr.splice(1, 2, "a", "b", "c");

console.log(arr);     // [1, "a", "b", "c", 4, 5]
console.log(removed); // [2, 3]

Summary: splice() is a universal "Swiss army knife" for arrays: it removes, adds and replaces elements right in the original array and returns an array of the removed values.

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