How to Use Strict Mode in JavaScript: Syntax and Examples

Use 'use strict'; at the top of a JavaScript file or function to enable strict mode, which helps catch common coding mistakes and unsafe actions. Strict mode changes how JavaScript runs by enforcing stricter parsing and error handling.

Syntax

Strict mode is enabled by placing the exact string 'use strict'; at the beginning of a script or a function. This tells JavaScript to run in a stricter way, preventing some silent errors and unsafe actions.

• Global strict mode: Put 'use strict'; at the very top of your JavaScript file.
• Function strict mode: Put 'use strict'; at the start of a function body to apply strict mode only inside that function.

javascript

'use strict';

function example() {
  'use strict';
  // function code here
}

Example

This example shows how strict mode catches an error when you try to use a variable without declaring it first. Without strict mode, this would create a global variable silently.

javascript

'use strict';

function test() {
  x = 10; // Error: x is not defined in strict mode
}

try {
  test();
} catch (e) {
  console.log(e.message);
}

Output

x is not defined

Common Pitfalls

Some common mistakes when using strict mode include:

• Forgetting to put 'use strict'; as the very first statement in a script or function.
• Using reserved keywords as variable names, which strict mode disallows.
• Assigning values to read-only properties or non-writable variables.

Here is an example showing a wrong and right way to declare variables in strict mode:

javascript

'use strict';

// Wrong: undeclared variable
// x = 5; // Throws ReferenceError

// Right: declare variable with let, const, or var
let x = 5;
console.log(x);

Output

5

Quick Reference

Feature	Effect in Strict Mode
Undeclared variables	Throws ReferenceError instead of creating global variables
Deleting variables or functions	Throws SyntaxError
Duplicate parameter names	Throws SyntaxError
Writing to read-only properties	Throws TypeError
Using reserved keywords as identifiers	Throws SyntaxError
Octal literals (e.g., 012)	Not allowed, throws SyntaxError

Key Takeaways

Add 'use strict'; at the top of your script or function to enable strict mode.

Strict mode helps catch common mistakes like undeclared variables and reserved keywords.

Always place 'use strict'; as the first statement to ensure it works correctly.

Strict mode makes JavaScript safer by throwing errors for unsafe actions.

Use strict mode to write cleaner and more reliable JavaScript code.