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Javascript Interview Questions & Answers

 


Basic Level:

1.What is JavaScript?

  • JavaScript is a high-level, interpreted programming language used to make web pages interactive.
2.What are the data types in JavaScript?
  • Primitive types: String, Number, Boolean, Null, Undefined
  • Object types: Object, Array, Function
3.Explain the difference between let, const, and var.
  • let and const are block-scoped, while var is function-scoped.
  • const is used for constant values that shouldn't be reassigned.
4.What is the DOM?
  • The Document Object Model represents the structure of a document as a tree of objects, allowing scripts to dynamically update content and style.
5.What is the difference between == and ===?
  • == checks for equality after type coercion, while === checks for equality without type coercion.

Intermediate Level:

  1. 1. Explain hoisting in JavaScript.
    • Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during compilation.
  2. 2.What is closure in JavaScript?
    • A closure is a function that retains access to variables from its outer (enclosing) scope even after the outer function has finished executing.
  3. 3.What is the event loop in JavaScript?
    • The event loop is the process that handles asynchronous operations by placing events in a queue and executing them in a single thread.
  4. 4.How does this keyword work in JavaScript?
    • this refers to the object it belongs to. In a function, this refers to the global object, but in a method, it refers to the object invoking the method.
  5. 5.Explain the concept of promises.
    • Promises are objects representing the eventual completion or failure of an asynchronous operation, providing a cleaner way to handle asynchronous code compared to callbacks.

  6. Advanced Level:

    1. 1.What is the purpose of the async and await keywords?

      • async is used to declare an asynchronous function, and await is used to wait for a Promise to resolve before continuing the execution.
    2. 2.Explain the concept of prototypal inheritance in JavaScript.
      • JavaScript uses prototype-based inheritance, where objects can inherit properties and methods from other objects through their prototype chain.
    3. 3.How does event delegation work?
      • Event delegation is a technique where a single event listener is attached to a common ancestor, and it can manage events for multiple child elements.
    4. 4.What are the differences between null and undefined?
      • null is an assigned value representing the absence of an object value, while undefined is a variable that has been declared but not assigned a value.
    5. 5.Explain the concept of a callback function and provide an example.
      • A callback function is a function passed as an argument to another function to be executed later. Example: setTimeout(function() { console.log("Hello, Callback!"); }, 1000);

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