How to insert an item into an array at a specific index using JavaScript

Matthew C.

—January 30, 2023

The Problem

You want to insert an item into an array at a specific index. For example, inserting an item at a specific index in a to-do list.

The Solution

There are various methods to insert an item into an array, broadly divided into those that change the original array and those that don’t.

Using the splice() Method

The splice() method is used to change an array by adding, removing, or replacing elements. This method modifies the original array.

const arr = ["walk the dog", "go shopping", "exercise"];
const index = 2;
const value = "go to the pharmacy";
arr.splice(index, 0, value);
console.log(arr); // "walk the dog", "go shopping", "go to the pharmacy", "exercise"

The first argument of the splice() method is the start index, which is the index at which to start changing the array. This is the only required argument. The second argument is the deleteCount. To add elements to an array, add each element as an additional argument. The splice() method returns an array of the deleted elements. If no elements are deleted, an empty array is returned.

If you are dealing with very large arrays, the splice() method is most likely to perform the best as it does not need to create a new array, unlike the other methods that are described below.

If you don’t want to modify the original array, you can also make a copy of the array before using the splice() method:

const arr = ["walk the dog", "go shopping", "exercise"];
const arrCopy = [...arr];
const index = 2;
const value = "go to the pharmacy";
arrCopy.splice(index, 0, value);
console.log(arrCopy); // "walk the dog", "go shopping", "go to the pharmacy", "exercise"

Using the slice() Method

The slice() method selects a portion of an array and returns a shallow copy of it. It does not modify the original array.

The slice() method has two optional arguments: The start argument is the zero-based index at which to start selecting a portion of the array. The end argument is the zero-based index at which to stop selecting a portion of the array. The slice() method extracts up to the end argument index, it does not include the array element at the end index.

The slice() method can be used along with spread syntax to insert an item into an array at a specific index:

const arr = ["walk the dog", "go shopping", "exercise"];
const index = 2;
const value = "go to the pharmacy";
const newArr = [...arr.slice(0, index), value, ...arr.slice(index, arr.length)];
console.log(newArr); // "walk the dog", "go shopping", "go to the pharmacy", "exercise"

Using a Loop

You can also use a for loop to loop through each item in an array, add each item to a new array, and insert an item at a specific index:

const arr = ["walk the dog", "go shopping", "exercise"];
const index = 2;
const value = "go to the pharmacy";
const newArr = [];
for (let i = 0; i < arr.length; i++) {
  if (i === index) {
    newArr.push(value);
  }
  newArr.push(arr[i]);
}

console.log(newArr); // "walk the dog", "go shopping", "go to the pharmacy", "exercise"

Using reduce()

You can create a custom function that uses reduce() to insert an element into an array at a specific index:

const arr = ["walk the dog", "go shopping", "exercise"];
const index = 2;
const value = "go to the pharmacy";

function insert(array, val, ind) {
  return ind >= arr.length
    ? array.concat(val)
    : array.reduce(
      (accumulator, currentValue, i) => accumulator.concat(i === ind ? [val, currentValue] : currentValue),
      [],
    );
}

console.log(insert(arr, value, index)); // "walk the dog", "go shopping", "go to the pharmacy", "exercise"

This method creates a new array; it does not change the original array.