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Find a Substring inside a String in JavaScript

In this post you’ll learn how to use JavaScript to find a substring inside a string.

Let’s assume we have a secret passcode 'bacon' and want to check if it exists in another string.

Here we’ll use findme to demonstrate the string we want to check:

const passcode = 'bacon';
const findme = `8sowl0xebaconxjwo98w`;

Visually we can see that findme contains 'bacon' but how do we get a yes/no answer in JavaScript?

We can introduce a new feature in ES6, the String.prototype.includes method, which will return us a Boolean value based on whether the substring was found:

const found = findme.includes(passcode);
// true
console.log(found);

🐛 Check the browser support for String includes! Either compile your code with Babel or provide a polyfill.

This is a really nice method and it gives us a straight-up Boolean answer - but it wasn’t always this way.

First, came String.prototype.indexOf - where we can ask for the index of the start of the string, should it match. If it does match, we get the index, or we get -1, so naturally our safety check does exactly this:

const index = findme.indexOf(passcode);

// true
console.log(index !== -1);

The -1 isn’t the best looking code, it feels a bit archaic - so prefer the use of String.prototype.includes where appropriate.

You could also use the bitwise operator ~ instead as a little shorthand, and we’ll leave it at that:

const index = !!~findme.indexOf(passcode);

// true
console.log(index);

The !!~ converts the resulting bitwise expression to a Boolean. It’s basically a fancy way of forcing indexOf to return us true or false in one line.

Summary

We’ve covered the new ES6 String.prototype.includes method and compared it to the previously-favoured String.prototype.indexOf method.

The different between the two is includes will return you a Boolean and indexOf will return you a number.

Typically includes is the go-to method unless you need to know the index!

🕵️‍♀️ Learn more about the indexOf method for finding a string inside a string

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