6. Introduction to Strict Mode

Historically, JavaScript was very permissive, allowing developers to make errors without throwing clear messages. Strict Mode was introduced in ECMAScript 5 (ES5) to make the language cleaner and safer.

How to Enable Strict Mode

Place the string 'use strict'; at the very top of your script file or inside a function.

javascript // In the file 'app.js' 'use strict';

// Code that might have been messy before will now throw errors.

// Example: Trying to use a variable without declaring it x = 10; // ERROR in strict mode (ReferenceError: x is not defined)

// Without strict mode, this would silently create a global variable 'x'.

Why Use Strict Mode?

1. Eliminates 'Bad Syntax': It catches common coding mistakes that lead to subtle bugs.
2. Improves Optimization: Engines can sometimes optimize strict mode code better.
3. Security: Prevents access to dangerous global variables.

Recommendation: Always use strict mode for new JavaScript code.