You've seen it in code created by friends or coworkers, maybe even used it yourself. That interesting line at the top of a JavaScript file: "use strict".

What an odd little line, sitting up there at the top by itself, seemingly not doing much. What is this line and why is it there? How is it affecting the code underneath it? What would happen if you just removed that line altogether.

Let's take a look at JavaScript's "Strict Mode". What is it, how does it work and why you should or shouldn't be using it in your day to day coding.