JavaScript Strict Mode (use strict) | Tutorial

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JavaScript Strict Mode (use strict)

JavaScript strict mode runs under stricter conditions.

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Using the "use strict" Directive

The "use strict" directive was introduced in JavaScript 1.8.5 (ECMAScript5).

It is not a statement, but a literal expression that is ignored in older versions of JavaScript.

The purpose of "use strict" is to specify that the code should be executed under strict conditions.

In strict mode, you cannot use undeclared variables.

Browsers that support strict mode: Internet Explorer 10+, Firefox 4+, Chrome 13+, Safari 5.1+, Opera 12+.

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Strict Mode Declaration

Strict mode is declared by adding the expression use strict; at the beginning of a script or function.

In the example, we can press F12 (or click "Tools > More Tools > Developer Tools") in the browser to open debug mode and view error messages.

You can also right-click and select "Inspect" to view it, as demonstrated in the Gif below:

Example

"use strict";
x = 3.14; // Error (x is not defined)

Try it yourself »

Example

"use strict";
myFunction();

function myFunction() {
  y = 3.14; // Error (y is not defined)
}

Try it yourself »

Declaring it inside a function makes it local (strict mode is only used within the function):

Example

x = 3.14; // No error
myFunction();

function myFunction() {
  "use strict";
  y = 3.14; // Error (y is not defined)
}

Try it yourself »

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Why use strict mode:

• Eliminates some unreasonable and sloppy aspects of JavaScript syntax, reducing some strange behaviors.
• Eliminates some unsafe aspects of code execution, ensuring code runs securely.
• Improves compiler efficiency and increases execution speed.
• Paves the way for future new versions of JavaScript.

"Strict mode" reflects a more reasonable, safer, and stricter development direction for JavaScript. Mainstream browsers, including IE 10, already support it, and many large projects have begun to fully embrace it.

On the other hand, the same code may produce different results in "strict mode"; some statements that can run in "normal mode" will not run in "strict mode." Mastering this content helps to understand JavaScript more deeply and makes you a better programmer.

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Restrictions of Strict Mode

Using undeclared variables is not allowed:

"use strict";
x = 3.14; // Error (x is not defined)

Try it yourself »

An object is also a variable.

"use strict";
x = {p1:10, p2:20}; // Error (x is not defined)

Try it yourself »

Deleting variables or objects is not allowed.

"use strict";
var x = 3.14;
delete x; // Error

Try it yourself »

Deleting functions is not allowed.

"use strict";
function x(p1, p2) {};
delete x; // Error