How to find elements with indexof in javascript

Learn how to check if elements or characters are present in Arrays or Strings with the indexOf method in javascript.

Arrays

indexOf method finds the index of the first matching element in the array and returns it. If element is not present then -1 is returned.
It searches the element with strict equality ===.

const list = [1, 2, 3, 4, 5, 6, 7];
const index = list.indexOf(1);
console.log(index);
//0

const index = list.indexOf(11);
//-1

The indexOf method takes two parameters as input.

Array.indexOf(element, startFrom)

element

• Required. Element whose index you are looking for.

startFrom

• Optional. Index from where the search of the element should start from.
• If startFrom is greater than Array’s length then -1 is returned.

Search element after the startFrom index

const list = [1, 2, 3, 4, 5, 6, 7];
const index = list.indexOf(4, 5);
console.log(index);
//-1

Search element before the startFrom index

const index = list.indexOf(5, 4);
console.log(index);
//4

Search element when startFrom index is greater than Array’s length

const index = list.indexOf(5, 9);
console.log(index);
//-1

String

We can only search for the position of characters with String’s indexOf method. It works same as the Array’s indexOf method.

const str = 'I am Prashant Yadav';
const index = str.indexOf('a');
console.log(index);
//2

String.indexOf(characters, startFrom)

characters

• Required. Character whose position you want to find.

startFrom

• Optional. A numeric value from where you want the search to start from.
• If startFrom is less than 0 then the search will start from 0 and if it is greater than String’s length then the search will begin at String.length.

You can search the index of white space as well.

const str = 'I am Prashant Yadav';
const index = str.indexOf(' ');
console.log(index);
//1

Search the index after the fourth character.

const str = 'I am Prashant Yadav';
const index = str.indexOf('a', 4);
console.log(index);
//7

Searching with the negative index

const str = 'I am Prashant Yadav';
const index = str.indexOf('a', -1);
console.log(index);
//2

Searching with the index greater than string’s length

const str = 'I am Prashant Yadav';
const index = str.indexOf('a', str.length+1);
console.log(index);
//-1