โ† flaviocopes.com

How to get the index of an item in a JavaScript array

Published Aug 19 2020

Psssst! The 2023 WEB DEVELOPMENT BOOTCAMP is starting on FEBRUARY 01, 2023! SIGNUPS ARE NOW OPEN to this 10-weeks cohort course. Learn the fundamentals, HTML, CSS, JS, Tailwind, React, Next.js and much more! โœจ

Suppose you have the value of an item which is contained in an array, and you want to get its index.

How can you get it?

If the item is a primitive value, like a string or number, you can use the indexOf method of an array:

const letters = ['a', 'b', 'c']

const index = letters.indexOf('b')

//index is `1`

Remember that the index starts from the number 0

If the item is an object, you canโ€™t use this way, because if you try doing:

const letters = [
  {
    letter: 'a',
  },
  {
    letter: 'b',
  },
  {
    letter: 'c',
  },
]

const index = letters.indexOf({
  letter: 'b',
})

index will be -1 which means the item was not found. Because objects are compared by reference, not by their values (differently for primitive types). The object passed to indexOf is a completely different object than the second item in the array.

You can use the findIndex value like this, which runs a function for each item in the array, which is passed the element, and its index. Returning from it will assign the return value to the return value of findIndex:

const letters = [
  {
    letter: 'a',
  },
  {
    letter: 'b',
  },
  {
    letter: 'c',
  },
]

const index = letters.findIndex((element, index) => {
  if (element.letter === 'b') {
    return true
  }
})

//index is `1`


I write books for developers ๐Ÿ‘‡๐Ÿผ

© 2023 Flavio Copes Flavio Copes made in Italy ๐Ÿ‡ฎ๐Ÿ‡น using Notion to Site Notion to Site