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

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 can’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`