Add JS string contains substring

blog_posts/javascript-string-contains-substring.md

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+---
+title: Check if a string contains a substring in JavaScript
+shortTitle: String contains substring
+type: story
+tags: javascript,string
+expertise: beginner
+author: chalarangelo
+cover: blog_images/sea-view-2.jpg
+excerpt: Let's have a look at how to check if a string contains a substring in JavaScript.
+firstSeen: 2022-07-27T05:00:00-04:00
+---
+
+### String.prototype.includes()
+
+The most straightforward substring search option is `String.prototype.includes()`, introduced in ES6. It's a simple method that returns a boolean value depending on whether the string contains the substring or not.
+
+```js
+const str = 'Hello world';
+str.includes('world'); // true
+str.includes('foo'); // false
+```
+
+### String.prototype.indexOf()
+
+Another option is `String.prototype.indexOf()`, which can be preferable if you need to support legacy browsers. As this methods returns the index of the first occurrence of the substring, you need to compare it to `-1` to know if the substring was found or not.
+
+```js
+const str = 'Hello world';
+str.indexOf('world') !== -1; // true
+str.indexOf('foo') !== -1; // false
+```
+
+### Case-insensitive substring search
+
+Both methods presented so far are case-sensitive. If you need to search for a substring that is case-insensitive, you can use `String.prototype.toLowerCase()` to convert both strings to lowercase. Then you can compare them, using any of the previous methods.
+
+```js
+const str = 'Hello world';
+const token = 'WORLD';
+str.toLowerCase().includes(token.toLowerCase()); // true
+```