JS Strict Mode

JavaScript Strict Mode: Catching Silent Bugs Early

JavaScript strict mode ("use strict";) puts the engine into a stricter execution mode that turns silent mistakes into loud runtime errors. It prevents accidental global variables, blocks unsafe assignments, and helps JavaScript engines optimize code faster.

What Changes Under Strict Mode

In classic sloppy mode, assigning a typo like usreName = "Ada" silently creates a global property on window. Under strict mode, that line immediately throws a ReferenceError so you catch the bug during testing.

Strict mode also throws a TypeError when you assign to read-only properties, delete plain variables, or pass duplicate parameter names like function add(a, a).

Where Strict Mode Is Automatic

You do not need to write "use strict"; inside ES modules (import/export) or inside ES6 class bodies, because JavaScript enables strict mode there automatically. Keep "use strict"; at the top of standalone classic scripts.

"use strict";

function buildOrder(price, quantity) {
  const subtotal = price * quantity;
  return subtotal;
}

// Attempting to modify a frozen object throws TypeError in strict mode
const settings = Object.freeze({ currency: "USD" });
try {
  settings.currency = "EUR";
} catch (err) {
  console.log("Caught strict mode error:", err.name);
}

console.log("Order total:", buildOrder(15, 3));

Common Mistakes to Avoid

• Placing "use strict"; below other statements: The directive only works if it appears as the very first statement of a script or function body.
• Expecting this inside standalone functions to be window: In strict mode, calling a plain function sets this to undefined instead of leaking the global window object.
• Mixing strict and non-strict concatenated files: Wrapping concatenated bundles inside an IIFE or switching to ES modules keeps strict boundaries clean.

Frequently Asked Questions

Do ES6 modules and React components run in strict mode automatically?

Yes, every ES module file using import or export and every class block runs in strict mode by default without needing the string directive.

Does "use strict"; affect browsers that do not support it?

All browsers released over the last decade support strict mode, and older engines simply treated "use strict"; as a harmless unused string expression.

Can I turn strict mode off halfway through a file?

No, once enabled for a script or function scope, strict mode applies to the entire script or function and cannot be disabled inside nested blocks.