We use JavaScript extensively at SourceLair to build the client-side applications that run in our customers' browsers. For example, https://www.sourcelair.com consists of tens of thousands of lines of JavaScript code that run in a browser.
A great place to get started with JavaScript is Codecademy's Learn JavaScript course: https://www.codecademy.com/learn/introduction-to-javascript.
All the JavaScript documentation you will need is available at the Mozilla Developer Network website:
- Generic JavaScript documentation: https://developer.mozilla.org/en-US/docs/Web/JavaScript
- Browser JavaScript documentation: https://developer.mozilla.org/en-US/docs/Web/Reference/API
An excellent book to read about JavaScript is Eloquent JavaScript by Marijn Haverbeke and it's available for free at https://eloquentjavascript.net/.