"use strict"; Defines that JavaScript code should be executed in “strict mode”.

Strict mode makes several changes to normal JavaScript semantics:

  1. Eliminates some JavaScript silent errors by changing them to throw errors.
  2. Prohibits some syntax likely to be defined in future versions of ECMAScript.

See transitioning to strict mode, if you want to change your code to work in the restricted variant of JavaScript.

Browsers not supporting strict mode will run strict mode code with different behavior from browsers that do, so don’t rely on strict mode without feature-testing for support for the relevant aspects of strict mode. Strict mode code and non-strict mode code can coexist, so scripts can opt into strict mode incrementally.

Declaring Strict Mode

Strict mode is declared by adding “use strict”; to the beginning of a script or a function.

Declared at the beginning of a script, it has global scope (all code in the script will execute in strict mode):


"use strict";
x = 3.14;       // This will cause an error because x is not declared

Strict mode for functions

Likewise, to invoke strict mode for a function, put the exact statement "use strict"; (or 'use strict';) in the function’s body before any other statements.

"use strict";

function myFunction() {
  y = 3.14;   // This will also cause an error because y is not declared

The “use strict”; Syntax

The syntax, for declaring strict mode, was designed to be compatible with older versions of JavaScript.

Compiling a numeric literal (4 + 5;) or a string literal (“John Doe”;) in a JavaScript program has no side effects. It simply compiles to a non existing variable and dies.

So "use strict"; only matters to new compilers that “understand” the meaning of it.

Why Strict Mode?

Strict mode makes it easier to write “secure” JavaScript.

Strict mode changes previously accepted “bad syntax” into real errors.

As an example, in normal JavaScript, mistyping a variable name creates a new global variable. In strict mode, this will throw an error, making it impossible to accidentally create a global variable.

— — —

What does “use strict” do in JavaScript, and what is the reasoning behind it?

This article about Javascript Strict Mode might interest you: 

John Resig – ECMAScript 5 Strict Mode, JSON, and More

To quote some interesting parts:

Strict Mode is a new feature in ECMAScript 5 that allows you to place a program, or a function, in a “strict” operating context. This strict context prevents certain actions from being taken and throws more exceptions.


Also note you can apply “strict mode” to the whole file… Or you can use it only for a specific function (still quoting from John Resig’s article):

// Non-strict code...

  "use strict";

  // Define your library strictly...

// Non-strict code... 

Which might be helpful if you have to mix old and new code 😉

So, I suppose it’s a bit like the "use strict" you can use in Perl (hence the name?): it helps you make fewer errors, by detecting more things that could lead to breakages.

Strict mode is now supported by all major browsers.

Inside native ECMAScript modules (with import and export statements) and ES6 classes, strict mode is always enabled and cannot be disabled.


Leave a Reply

Your email address will not be published. Required fields are marked *