JavaScript Strict Mode: What use strict Actually Does

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.

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

• Prohibits deleting variables, functions or objects.

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

• Restricts this in functions. Without strict mode:

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

With strict mode:

"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.