Strict mode

The strict mode introduces a more restricted variant of JavaScript into your module, function or classes. It's an opt-in mechanism and it doesn't have to be used within the whole application - although it's recommended. You can turn in on e.g. in one function when the rest can still be written in a "normal" JavaScript. It may be helpful if you want to migrate a legacy project. The mode makes several changes to normal JavaScript semantics:

• introduces early errors - throws an error for errors that aren't strictly syntax ones but are thrown at compile time. Such errors without the mode might be treated as a mistake without an error, so the program runs, but it may create worse problems in the future. E.g.⁠

1⁠'use strict';
2⁠let variable;
3⁠vriable = 10; // There's a mispelling, so this variable doesn't exist. Without the strict mode on, this variable would be created, but in this case, a ReferaceError is thrown, preventing us from a bug
4

• speeds up the code - it fixes mistakes that may have an impact on JavaScript engines' optimalizations
• make the code more secure

Strict mode is turned on by default for ES6 modules and classes.