Top 25 interview questions of javascript

Fundamental Concepts

1. What is JavaScript, and how does it differ from Java?

   • JavaScript is a high-level, interpreted programming language used primarily for client-side web development, but it can also be used for server-side development with Node.js. It's dynamic, flexible, and known for its event-driven programming model.

   • Java is a general-purpose, object-oriented programming language that is compiled into bytecode and can run on any platform with a Java Virtual Machine (JVM). It's known for its strong typing, platform independence, and enterprise applications.

2. Explain the difference between var, let, and const.

   • var: Function-scoped, can be re-declared and re-assigned within the same scope.

   • let: Block-scoped, cannot be re-declared but can be re-assigned within the same scope.

   • const: Block-scoped, cannot be re-declared or re-assigned once initialized.

3. What is hoisting in JavaScript?

   • Hoisting is a JavaScript mechanism that moves variable declarations to the top of their scope, regardless of where they are actually placed. Function declarations are also hoisted to the top.

4. Explain closures in JavaScript.

   • A closure is a function that has access to variables in its outer (enclosing) function, even after the outer function has returned. This is possible because closures create a new scope that retains references to variables from the outer scope.  

5. What is the difference between null and undefined?

   • null: A value that represents the absence of an object.

   • undefined: A value that indicates a variable has not been assigned a value yet.

Object-Oriented Programming

1. Explain prototype inheritance in JavaScript.

   • JavaScript uses prototype-based inheritance, where objects inherit properties and methods from their prototype object. A prototype chain is formed, allowing objects to access properties and methods from their parent prototypes.

2. What is the difference between this keyword in different contexts?

   • Function context: this refers to the object that the function is called on.

   • Object method context: this refers to the object itself.

   • Constructor context: this refers to the new object being created.

   • Global context: this refers to the global object (e.g., window in a browser).

3. How do you create an object in JavaScript?

   • Object literal: let person = { name: "Alice", age: 30 };

   • Constructor function: function Person(name, age) { this.name = name; this.age = age; }

   • Class syntax: class Person { constructor(name, age) { this.name = name; this.age = age; } }

DOM Manipulation

1. How do you select elements in the DOM using JavaScript?

   • getElementById: Selects an element by its ID.

   • getElementsByTagName: Selects elements by their tag name.

   • getElementsByClassName: Selects elements by their class name.

   • querySelector: Selects the first element that matches a CSS selector.

   • querySelectorAll: Selects all elements that match a CSS selector.  

2. What is the difference between innerHTML and textContent?

• innerHTML: Sets or gets the HTML content of an element, including child elements and their attributes.

• textContent: Sets or gets the plain text content of an element, excluding child elements and their attributes.

Events and Asynchronous Programming

1. Explain event bubbling and event capturing.

• Event bubbling: Events propagate up the DOM hierarchy from the target element to its ancestor elements.

• Event capturing: Events propagate down the DOM hierarchy from the outermost element to the target element.

1. What is the purpose of the addEventListener method?

• The addEventListener method attaches an event handler to an element, allowing you to specify the event type and the function to be called when the event occurs.

1. How do you handle asynchronous operations in JavaScript?

• Callbacks: Functions that are passed as arguments to other functions and are executed when the asynchronous operation completes.

• Promises: Objects that represent the eventual completion (or failure) of an asynchronous operation.

• async/await: Syntax that makes asynchronous code look more like synchronous code.

Advanced Topics

1. What is the difference between synchronous and asynchronous code execution?

• Synchronous code: Executes line by line, blocking the execution of subsequent code until the current line is finished.

• Asynchronous code: Executes non-blocking operations, allowing the program to continue executing while waiting for the asynchronous operation to complete.

1. Explain the concept of a module in JavaScript.

• A module is a reusable unit of code that can be imported into other modules. JavaScript modules can be created using CommonJS, ES modules, or AMD.

1. What is the purpose of the strict mode in JavaScript?

• Strict mode is a mode that enforces stricter rules for JavaScript code, such as preventing certain errors and enforcing best practices.

1. What is the difference between a regular expression and a string?

• Regular expression: A pattern that matches a set of characters.

• String: A sequence of characters.

1. Explain the concept of a scope chain in JavaScript.

• A scope chain is the hierarchy of scopes that are accessible to a given piece of code. When a variable is referenced, the JavaScript engine searches the scope chain from the innermost scope to the outermost scope until it finds the variable.

1. What is the purpose of the typeof operator?

• The typeof operator returns a string indicating the type of the operand.

1. How do you create a custom event in JavaScript?

• You can create a custom event using the CustomEvent constructor.

Performance and Optimization

1. How can you improve the performance of your JavaScript code?

• Minimize DOM manipulations: Avoid frequent updates to the DOM, as they can be expensive.

• Use event delegation: Attach event handlers to a parent element and use event bubbling or capturing to handle events for child elements.

• Optimize JavaScript code: Minimize the number of calculations and avoid unnecessary operations.

• Use efficient data structures: Choose data structures that are appropriate for the task at hand.

1. What is the difference between == and === operators?

• ==: Performs type coercion before comparison.

• ===: Compares both value and type.

1. Explain the concept of garbage collection in JavaScript.

• Garbage collection is the process of automatically freeing memory that is no longer being used. JavaScript engines typically use a mark-and-sweep algorithm for garbage collection.

1. What is the purpose of the defer and async attributes for <script> tags?

• defer: The script will be executed after the page has finished parsing, but before the DOMContentLoaded event fires.

• async: The script will be executed as soon as it is available, without waiting for the rest of the page to be parsed.

1. What is the difference between a function expression and a function declaration?

• Function expression: A function that is assigned to a variable.

• Function declaration: A function that is declared using the function keyword.