? JavaScript Strict Mode

⚡ Quick Overview

Strict mode is a safer and more controlled version of JavaScript introduced in ECMAScript 5. It helps developers avoid silent errors, prevents accidental global variables, and disallows risky language features that can introduce bugs.

It is enabled by placing the directive "use strict"; at the top of a script or inside a function.

? Key Concepts

• Strict mode is optional and must be explicitly enabled.
• It can be applied globally or only inside a function.
• Using undeclared variables causes an error instead of creating globals.
• Invalid operations throw errors rather than failing silently.
• Some words become reserved and cannot be used as identifiers.
• this inside a normal function becomes undefined.

? Syntax and Theory

Strict mode works using a directive: a plain string expression at the beginning of code.

• It must appear as the first statement.
• Accepted formats are "use strict"; and 'use strict';.
• If placed incorrectly, it behaves like a normal string.

? Code Examples

"use strict";
x = 10;
ReferenceError: x is not defined

function example() {
"use strict";
y = 20;
}
ReferenceError: y is not defined

"use strict";
function test() {
console.log(this);
}
test();
Output: undefined

? Live Output and Explanation

? When to Use Strict Mode

• While learning JavaScript for better error feedback.
• In production code to prevent hidden bugs.
• In large projects to reduce unpredictable behavior.