JavaScript is a popular programming language used for web development, and its knowledge is highly sought after in job interviews.
While specific interview questions may vary depending on the role and company, here are ten common JavaScript topics that are frequently asked in job interviews:
1). Variables and Data Types: Questions about the variable declaration, scoping, and different data types such as strings, numbers, arrays, and objects.
Question:
What are the differences between let, const, and var in JavaScript?
Answer:
In JavaScript, let and const are block-scoped variables introduced in ECMAScript 6, while var is function-scoped.
let allows variable reassignment, while const creates a read-only constant. var has function scope and can be hoisted, whereas let and const have block scope and are not hoisted.
2). Functions: Topics related to function declarations, function expressions, anonymous functions, arrow functions, and their usage.
Question:
What is the difference between function declarations and function expressions in JavaScript?
Answer:
Function declarations are hoisted and can be used before they are declared. Function expressions, on the other hand, are not hoisted and must be defined before they are used.
3). DOM Manipulation: Questions on interacting with the Document Object Model (DOM), including selecting elements, modifying attributes, handling events, and dynamically creating elements.
Question:
How can you select elements in the DOM using JavaScript?
Answer:
JavaScript provides several methods for selecting DOM elements, such as getElementById, querySelector, and getElementsByClassName. These methods allow you to select elements by their ID, CSS selector, or class name.
4). Closures: Understanding closures, lexical scope, and their practical use cases.
Question:
What is a closure in JavaScript and why are they useful?
Answer:
A closure is created when a function has access to variables from its outer scope, even after the outer function has finished executing. Closures are useful for creating private variables, implementing data encapsulation, and preserving the state of variables.
Closures are useful in scenarios where you want to preserve the state of variables or create private variables in JavaScript. They allow you to encapsulate data within a function and access that data even after the function has completed execution.
5). Asynchronous JavaScript: Topics related to callbacks, promises, async/await, and handling asynchronous operations in JavaScript.
Question:
Explain the concept of promises in JavaScript and how they are used for handling asynchronous operations.
Answer:
Promises are objects that represent the eventual completion or failure of an asynchronous operation. They help in handling asynchronous code by providing a cleaner syntax and allowing the chaining of multiple asynchronous operations.
6). Object-Oriented Programming: Questions about object creation, prototypes, inheritance, and object-oriented concepts in JavaScript.
Question:
How can you create objects and implement inheritance in JavaScript?
Answer:
In JavaScript, objects can be created using object literals, constructor functions, or the class syntax introduced in ECMAScript 6.
Inheritance can be achieved through prototypal inheritance, where objects inherit properties and methods from their prototype chain.
7). Error Handling: Knowledge of handling and throwing errors, try-catch blocks, and error propagation.
Question:
How do you handle errors in JavaScript?
Answer:
In JavaScript, errors can be handled using try...catch blocks. The code that might throw an error is placed within the try block, and if an error occurs, it is caught in the catch block, where you can handle or log the error.
8). ES6+ Features: Questions regarding modern JavaScript features introduced in ECMAScript 6 and later, such as arrow functions, let and const, destructuring, spread operators, and modules.
Question:
Name some new features introduced in ECMAScript 6 (ES6) and later versions of JavaScript.
Answer:
Some ES6+ features include arrow functions, template literals, destructuring assignment, spread syntax, let and const for variable declarations, modules with import and export, and enhanced object literals.
9). Scope and Hoisting: Understanding variable hoisting, function hoisting, and lexical scope in JavaScript.
Question:
Explain the concept of hoisting in JavaScript.
Answer:
Hoisting is a JavaScript behavior where variable and function declarations are moved to the top of their containing scope during the compilation phase.
This means that you can use variables and functions before they are declared, although it’s recommended to declare them before using them for code clarity.
10). Array Methods and Higher-Order Functions: Familiarity with common array methods like map, filter, reduce, forEach, and understanding the concept of higher-order functions.
Question:
What are higher-order functions in JavaScript? Provide an example.
Answer:
Higher-order functions are functions that can take other functions as arguments or return functions as their results. An example of a higher-order function is map, which applies a given function to each element of an array and returns a new array with the transformed values.
//Variables and Data Types
// Example 1: Using let
let x = 10;
if (true) {
let x = 20;
console.log(x); // Output: 20
}
console.log(x); // Output: 10
// Example 2: Using const
const y = 30;
// y = 40; // Error: Assignment to constant variable.
console.log(y); // Output: 30
// Example 3: Using var
var z = 50;
if (true) {
var z = 60;
console.log(z); // Output: 60
}
console.log(z); // Output: 60
//Suyash Chandrakar