Strict mode in JavaScript

Strict mode in JavaScript is a special operating mode that helps write safer and better quality code. By enabling it, you activate additional checks and restrictions that prevent common errors.

What Does Strict Mode Change?

Prohibits using undeclared variables.

bash

"use strict";
x = 10; // Error: variable not declared

Prohibits deleting variables, functions or objects.

bash

"use strict";
delete x; // Error: cannot delete variable or function

Restricts this in functions. Without strict mode:

bash

function showThis() {
  console.log(this); // `this` refers to global object (window in browser)
}

With strict mode:

bash

"use strict";
function showThis() {
  console.log(this); // `this` will be undefined
}

Prevents using reserved words. Words like implements, interface, package cannot be used as variables or function names.

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.

Finished reading?

Practice Problems

Strict mode in JavaScript

#What Does Strict Mode Change?

Short Answer

What Does Strict Mode Change?