Strict Mode

JavaScript is a loosey-goosey language, as we've learned. As such, it's easy to make mistakes that are hard to catch until a user is emailing you a bug report.

Strict Mode is a way to opt-in to a more restrictive set of rules that help you catch errors earlier. It was introduced in ES5. Some of the big differences between strict mode and "normal" (sloppy) JavaScript are:

Eliminates some silent errors by throwing them
Fixes some mistakes that make it difficult for JS engines to optimize code. Strict mode code can sometimes run faster than identical code that's not in strict mode.
Prohibits some syntax likely to be defined in future versions of ECMAScript
In the browser, this is undefined in global scope

Long story short, when possible, strict mode is a good idea.

To enable strict mode, you just need to "use strict" at the top of your file:

"use strict";

// your code here

You can also enable strict mode for a single function:

function strictFunction() {
  "use strict";
  // your code here
}

Strict Mode in Modules

Here's the best part: you only need "use strict"; at the top of non-es6 modules. ES6 modules, which we will learn next, are always in strict mode by default. That goes for the browser and Node.js.

We'll go more in-depth on Node and browser modules in a moment.

Assignment

JavaScript normally lets you get away with all kinds of nonsense, so let's do some JavaScript shenanigans:

Create a new file called strict.js
Assign "moo!" to an undeclared variable and console.log that variable

message = "moo!";
console.log(message);

Run the script

node strict.js

This works just fine, and you should see "moo!" printed to the console! However, if we enable strict mode, this will throw an error instead of silently creating a global variable.

Enable strict mode by adding "use strict"; at the top of the file and run the script again. This time, you should see an error.

With strict.js still throwing an error, run and submit the CLI tests.