The "use strict"; directive in JavaScript enables strict mode, a restricted variant of JavaScript that helps developers write more reliable, secure, and error-free code. Introduced in ECMAScript 5 (ES5), strict mode enforces stricter parsing and error handling rules, reducing the chances of subtle bugs.

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What “use strict” Does

1. Prevents Undeclared Variables
   In non-strict mode, assigning a value to an undeclared variable automatically creates it as a global variable. Strict mode throws an error instead.Example:javascriptCopy code"use strict"; x = 10; // ReferenceError: x is not defined
2. Eliminates this Coercion
   In strict mode, this in a function called without an object is undefined instead of being coerced to the global object.Example:javascriptCopy code"use strict"; function showThis() { console.log(this); } showThis(); // undefined
3. Disallows Duplicate Parameter Names
   Strict mode prevents duplicate parameter names in function declarations, which can lead to ambiguity.Example:javascriptCopy code"use strict"; function myFunction(a, a) { // SyntaxError: Duplicate parameter name not allowed return a; }
4. Throws Errors for Writing to Read-Only Properties
   Assigning values to non-writable properties or objects throws an error in strict mode.Example:javascriptCopy code"use strict"; const obj = Object.freeze({ key: 42 }); obj.key = 99; // TypeError: Cannot assign to read-only property
5. Prohibits Octal Literals
   Strict mode disallows the use of octal literals (e.g., 0123) for better clarity.Example:javascriptCopy code"use strict"; let num = 0123; // SyntaxError: Octal literals are not allowed
6. Requires Secure eval
   Variables and functions declared inside eval() are not accessible outside its scope.Example:javascriptCopy code"use strict"; eval("var x = 2;"); console.log(x); // ReferenceError: x is not defined
7. Reserved Keywords for Future Use
   Strict mode reserves certain words like implements, interface, private, protected, and public for future JavaScript versions.Example:javascriptCopy code"use strict"; let public = "test"; // SyntaxError: Unexpected strict mode reserved word

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Why Use “use strict”?

1. Better Error Checking
   Strict mode helps catch common coding errors early, making debugging easier.
2. Avoids Accidental Globals
   Mistakenly creating global variables can lead to unexpected behaviors in larger applications. Strict mode prevents this.
3. Improves Performance
   Some JavaScript engines optimize strict mode code better since it eliminates certain ambiguities.
4. Prepares for Future JavaScript Versions
   Strict mode enforces rules that align with modern JavaScript standards, making your code more future-proof.

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How to Use “use strict”?

1. Globally (Entire Script): Place "use strict"; at the top of a file to enforce strict mode for the entire script.javascriptCopy code"use strict"; let x = 10;
2. Locally (Specific Functions): You can enable strict mode within a specific function instead of the entire script.javascriptCopy codefunction strictFunction() { "use strict"; let y = 20; }

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Drawbacks of “use strict”

• Older JavaScript code might not work properly in strict mode because it enforces modern rules.
• Strict mode applies only to the scope where it’s enabled, leading to inconsistencies if not applied consistently.

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Conclusion

The "use strict"; directive is a valuable tool for writing cleaner and safer JavaScript code. By enforcing stricter rules, it helps developers avoid common pitfalls and future-proofs their applications. For best practices, consider using strict mode in all your modern JavaScript projects.