function createCounter() let count = 0; // Private variable return function() count++; return count; ; const counter = createCounter(); console.log(counter()); // 1 console.log(counter()); // 2 Use code with caution.
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Hoisting is JavaScript's default behavior of moving declarations to the top of the current scope before execution. var variables are hoisted and initialized with undefined . function createCounter() let count = 0; // Private
| Topic | Question | Answer | | :--- | :--- | :--- | | | What is JavaScript? | A scripting language that enables interactive web pages and dynamic content on the client side. | | Data Types | What are the different data types in JavaScript? | Primitive: String, Number, BigInt, Boolean, Undefined, Null, Symbol. Non-primitive: Object (Array, Function, Date, etc.). | | Variables | What's the difference between var , let , and const ? | var is function-scoped, while let and const are block-scoped. const cannot be reassigned; let can. | | Hoisting | What is hoisting in JavaScript? | A mechanism where variable and function declarations are moved to the top of their scope before code execution. | | Closures | What is a closure in JavaScript? | An inner function that has access to its outer (enclosing) function's variables, even after the outer function has returned. | | this Keyword | What is the this keyword in JavaScript? | A reference to the current execution context. Its value depends on how a function is invoked (as a method, a function, etc.). | | Equality | What is the difference between == and === ? | == compares values after type coercion (e.g., 1 == '1' is true ), while === compares both value and type without coercion (e.g., 1 === '1' is false ). | | Asynchronous JS | How does JavaScript handle asynchronous operations? | Through the event loop, callbacks, Promises, and the async/await syntax, which allows for non-blocking code execution. | | Arrays | What is the difference between map() and forEach() ? | map() creates a new array by applying a function to every element, while forEach() executes a function on each element but does not return a value. | | Prototypes | How does prototypal inheritance work? | Objects can inherit properties and methods from other objects via a prototype chain. When a property is accessed, JavaScript walks up this chain until it finds it. | | Topic | Question | Answer | |