Skip to main content Accessibility Feedback

Checking for an item in a string or array after a specific index with vanilla JS

This week, we’ve looked at how to check for text in a string and how to check for an item in an array.

What if you wanted to do those things, but you wanted to start looking at a certain index?

Let’s learn how.

Specifying a starting position with the includes() and indexOf() methods

The includes() and indexOf() methods for both strings and arrays let you specify a starting position as an optional second argument.

If you pass that argument in, the method will start looking for the substring or item at that index.

Here’s an example with a string.

var hitchhiker = 'The answer to the ultimate question of life, the universe, and everything';

// returns true

// returns false, because "question" starts at index 27
hitchhiker.includes('question', 42);

// returns 27

// returns -1 for the same reason
hitchhiker.indexOf('question', 42);

And here’s one with an array.

var wizards = ['Hermione', 'Neville', 'Harry Potter', 'Dumbledore'];

// returns true

// returns false, because "Neville" has an index of 1
wizards.includes('Neville', 3);

// returns 1

// returns -1 for the same reason
wizards.indexOf('Neville', 3);

When would you need this?

Honestly, I’m not sure. I’ve never had a need for this one, but in case you might, it’s good to have the option.