Hello, you don’t need to download JavaScript. You can just write it as plain text. It can't do anything until you have a JavaScript engine. This is a program which reads and interprets the code and produces an output.
Since JavaScript is mainly a language for the web, all web browsers have a JavaScript engine. However, you can’t simply load a JavaScript file into a web browser. It has to be attached to an HTML page. When you load an HTML page in the web browser, the browser will parse the page and then interpret the JavaScript.