This content originally appeared on DEV Community and was authored by nexgismo

Want to write JavaScript that’s easier to debug, less error-prone, and more secure?
Strict Mode might be the single line that changes how you write JS forever.

What Is Strict Mode in JavaScript?

Strict mode is a way to opt into a restricted variant of JavaScript introduced in ECMAScript 5. It helps eliminate silent bugs and forces better coding discipline.

You enable it by placing:

'use strict';

at the top of a script or function.

Why Use Strict Mode?

Feature	Without Strict Mode	With Strict Mode
Undeclared variables	Allowed	ReferenceError
this in functions	Refers to window	undefined
Duplicate parameters	Allowed	SyntaxError
with statement	Allowed	Disallowed
Read-only assignments	Ignored	TypeError

Strict mode can:

• Prevent accidental globals
• Make this more predictable
• Ban confusing or insecure features

How to Enable Strict Mode

Globally

'use strict';
let points = 100;

Locally (function scope)

function calculate() {
  'use strict';
  let result = 50;
}

ES6 modules and class bodies are in strict mode by default.

Real-World Example

Without strict mode:

function addTax(price) {
  tax = price * 0.18; // Creates a global variable!
  return tax;
}

With strict mode:

function addTax(price) {
  'use strict';
  let tax = price * 0.18;
  return tax;
}

Now you’ll catch undeclared variables before they cause damage.

Automatic Strict Mode in ES6

Strict mode is automatically applied in:

• ES6 modules (import / export)
• Class definitions

No need to manually declare it in those places.

How It Helps Security

Strict mode blocks many insecure or legacy features:

• Global scope pollution
• Unsafe assignments
• Silent failure during reassignment
• Accidental overwriting of built-ins

When Should You Use It?

Situation	Use Strict Mode?
New apps	Absolutely
Large teams	Yes
Legacy code	Use in isolated scopes
ES6 modules	Already enabled

Conclusion

Strict Mode may seem optional, but it’s a best practice that improves code reliability, security, and maintainability.

It’s just one line — but it can save hours of debugging and prevent painful production bugs.

Full Visual Blog + FAQ + PDF Download:
Read on NexGismo.com

Have you had a bug that strict mode could’ve caught early? Share your story below

#javascript #webdevelopment #cleanCode #codingtips #softwareengineering

This content originally appeared on DEV Community and was authored by nexgismo