For many years, JavaScript evolved without compatibility issues. New features were being added to the language while maintaining old functionality.

In 2009, ECMAScript 5 (ES5) was released. It added new powerful features to the language, but it also modified some of the existing ones.

To keep the old code working, most of this modifications are off by default. You need to explicitly enable them using a specific command: "use strict".

Strict Mode is a new feature introduced in ECMAScript 5 that allows you to place code in a "strict" context.

This can be extremely useful, since it prevents certain actions from being taken or throws errors when an 'unsafe' action is taken (like accessing the global object). Basically, it's a JavaScript feature that helps you make fewer errors, by detecting potential errors in advance that could lead to major problems in the future.

You can apply "strict mode" to the entire file you are working on, by placing it at the top of a script or using it inside a specific function. This is great because you can now define complete JavaScript libraries in a strict mode without affecting outside code. Enabling strict mode is as simple as adding "use strict" string (you can also use single quotes), like in the code block below:

// Non-strict code...

(function () {
  "use strict";

  // code running in strict mode...
})();

// Non-strict code...

That's it! No new syntax is introduced in order to enable strict mode.

Now that we have given an introduction to strict mode, let's see what actually changes when you put a script into strict mode.

A great example is using it for variables. Previously, an attempt to assign dog = 'Rex', where dog hasn't been defined, would have assigned the value to the dog property of the global object (e.g. window.dog).

In strict mode, if the variable hasn't been defined will fail.

Keep in mind that once you enter the strict mode, there is no way of going back. There's no way to cancel "use strict" and there is no command like "disable use strict".

If you are writing 'modern' JavaScript, with import and export statements and ES6 classes, then strict mode enabled by default and it cannot be disabled. Strict mode is now supported by all major browsers, so there are no excuses not use it and save yourself from potential future headaches.