A language may be implemented as an interpreter, but it could be implemented as an ahead-of-time compiler, or a mix of both. Here we'll actually start looking at some code, and while doing so, explore what actually happens when you run some JavaScript in your page. Programming languages are technically just doing complicated math very, very quickly. Out of the box working, easier and cleaner. Instead of including JavaScript in your HTML, use a pure JavaScript construct. This evolution has prompted the development of JIT compilers, which help optimize execution. For instance, it could translate the codes from JavaScript to C++. JavaScript Dynamic client-side scripting. As a last step, the generated AST either gets interpreted or compiled to assembly. Suppose you have the following program. With a script you can use an ftp tool and edit the text directly and then save it. If you are using JavaScript to manipulate elements on the page (or more accurately, the Document Object Model), your code won't work if the JavaScript is loaded and parsed before the HTML you are trying to do something to. None of these two are correct. Surely the speed increases from being compiled would be useful for heavy load sites? Did you add your