Strict mode in JavaScript

#javascript #webdev #strictmode #beginners

What is strict mode?

Strict mode was introduced in ECMAScript5 also known as ES5 in 2009.
Adding strict mode to JavaScript code allows your code to be checked more strictly.
Allows programmer to write more secure JavaScript code.

Benefits of Strict Mode

Using strict mode in JavaScript code throws visible errors in developer console which would otherwise fail silently (shown in the examples below).
Forbids programmers to use certain syntax.
In some cases using strict mode in code can make the code to run faster.

How to enable strict mode in JavaScript?

To enable strict mode write "use strict" or 'use strict' at the beginning of JavaScript code.
The strict mode can also be used for a function.
To enable strict mode in function, "use strict" or 'use strict' must be the very first line in function body.

Common mistakes that occur in non-strict mode

Using reserved words as variable names

Wait!! Isn't "let" a reserved word in the JavaScript? Therefore it mustn't be used as a variable name according to naming an identifier rules. Here in the above code snippet JavaScript fails silently instead of throwing an error.
But, if we use strict mode, JavaScript engine will throw an error-

Using variables before declaring them

Using strict mode gives Reference Error.
To know more about the strict mode, checkout the documentation by MDN

Conclusion

That will be all from my side. I hope, this article gave you a basic idea about the strict mode.

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Top comments (5)

Jon Randy 🎖️ • Oct 15 '21

Modules are strict mode by default

Abhishek Rath • Oct 15 '21

Yeah, the strict mode became default after the introduction of ES6. I forgot to mention that in the post.
Thanks for sharing!!

Piotr Lewandowski • Oct 15 '21

"strict mode became default after the introduction of ES6." - no, it did not. Like Jon above said: MODULES introduces in ES2015/ES6 are strict by default. Everything else is still as it was - "sloppy mode" -until you add 'use strict'; into your code ;)