Add an Item to the Beginning of an Array in JavaScript or Node.js

Arrays are a commonly used data structure in JavaScript. When interacting with arrays, you likely want to manipulate them by adding new items. In some situations, you may want to add items to the beginning of an array.

This tutorial shows you three ways on how to prepend items to an array.

Node.js Series Overview

  1. Get a File’s Created Date
  2. Get a File’s Last Modified/Updated Date
  3. How to Create an Empty File
  4. Check If a Path or File Exists (Coming soon)
  5. How to Rename a File (Coming soon)
  6. Check If a Path Is a Directory (Coming soon)
  7. Check If a Path Is a File (Coming soon)
  8. Retrieve the Path to the User’s Home Directory (Coming soon)
  9. How to Touch a File (Coming soon)

3 Ways Adding Items to the Beginning of a JavaScript Array

JavaScript supports different ways of adding items to the beginning of an array. You can use the native Array#unshift method, create a new array in combination with spreading existing items into the new array or create a new array by concatenating two arrays.

Prepend Items to an Existing Array

The Array#unshift method adds one or more items to the beginning of an array.

Notice: Array#unshift returns the new length of the array, not the array containing the new item(s):

const ids = [1, 2]

const newArrayLength = ids.unshift(3)  
// 3
// the value of "ids" is [3, 1, 2]

// the value of "ids" is [4, 3, 1, 2]

// or prepend multiple values

ids.unshift(5, 6, 7)  
// the value of "ids" is [5, 6, 7, 4, 3, 1, 2]

Add and Spread Items Into a New Array

You can also create a new array and add the new items at the beginning. This is a short and concise way in combination with the spread operator to add the values of an existing array:

const ids = [1, 2, 3]

const newIds = [4, ...ids]  
// [4, 1, 2, 3]

const newIds = [4, 5, 6, ...ids]  
// [4, 5, 6, 1, 2, 3]

Concat(enate) Arrays Items

You may also create a new array of the items you want to prepend to the existing array. Then, concatenate the existing items to the end of the new array. This keeps the new items in the front and appends the existing ones to the end:

const ids = [1, 2, 3]

const newIds = [4, 5, 6].concat(ids)  
// the value of "newIds" is [4, 5, 6, 1, 2, 3]

All three ways allow you to prepend one or more items to the beginning of an array. Enjoy!

What Should You Use?

Well, use what you like best!

I prefer Array#concat when building an array pipeline by chaining further operations, like Array#map, Array#filter, or Array#find.

In situations where I only want to prepend the item without additional operations, I’m using the spread operator.

In terms of performance: I don’t really care about performance optimizations on my side. I’m relying on the JavaScript runtime to optimize the code for me. This gives me the freedom of writing the code I like to read 😃

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