Top 10 Javascript Interview Questions and Answers

1. What is JavaScript?

JavaScript is a popular web scripting language and is used for client-side and server-side development. The JavaScript code can be inserted into HTML pages that can be understood and executed by web browsers while also supporting object-oriented programming abilities.

2. What is differences between Java and JavaScript?

Java is a complete programming language. In contrast, JavaScript is a coded program that can be introduced to HTML pages. These two languages are not at all inter-dependent and are designed for different intent. Java is an object-oriented programming (OOPS) or structured programming languages like C++ or C, whereas JavaScript is a client-side scripting language.

Java creates applications that run in a virtual machine or browser but JavaScript code is run on a browser only.

Java source code needs a compiler before it can be ready to run in realtime but JavaScript does not need compilation before running the application code.

3. What are undeclared and undefined variables?

Undeclared variables are those that do not exist in a program and are not declared. If the program tries to read the value of an undeclared variable, then a runtime error is encountered.

Undefined variables are those that are declared in the program but have not been given any value. If the program tries to read the value of an undefined variable, an undefined value is returned.

4. What are JavaScript Data Types?

These are the different types of data that JavaScript supports:

• Boolean - For true and false values
• Null - For empty or unknown values
• Undefined - For variables that are only declared and not defined or initialized
• Number - For integer and floating-point numbers
• String - For characters and alphanumeric values
• Object - For collections or complex values
• Symbols - For unique identifiers for objects

5. What are the different types of errors in JavaScript?

JavaScript errors

In any programming language, errors occur. There are basically two types of errors in programming. These are:

1. Program Error:
   This is an error encountered by the program that might require error handlers to manage. An example could be network disconnection, timeout error, HTTP response errors, etc.
2. Developer Error:
   This is an error caused by the programmer. Examples include syntax errors, logical errors, semantic errors, etc.

Error Types:

1. Syntax error (SyntaxError): This is done when you use an already pre-defined syntax in the wrong way.
2. Reference Error (ReferenceError): this occurs when a variable reference can’t be found or is not declared.
3. Type Error (TypeError()): This is an error caused by misusing a value outside its own data type scope.
4. Evaluation Error (EvalError()): This is no longer thrown by the current JavaScript Engine or EcmaScript Specification. However, it still exists for backward compatibility. It is called when you use the eval()backward function.
5. RangeError (RangeError()): This error is caused when there is a need for an expected range of values.
6. URI Error(URIError()): This is called whenever a wrong character(s) is used in any URI function.

6. What do you mean by the ‘this’ keyword in JavaScript?

In the case of Java, the ‘This’ keyword is used to point to the current object, but in JavaScript, the ‘This’ keyword refers to them from where it was called.

In other Words, we could say that the” this” keyword refers to the object it belongs to.

So, the ‘this’ keyword has different values according to or depending upon where it is used.

1. If we use it in a method then, this refers to the owner object.
2. If it is alone, then this refers to the Global object.
3. If it is used in a function, then this refers to the global object
4. If it is used in a function, in strict mode, then this remains undefined.
5. In an event, this refers to that respective element that receives the event.

Call() and apply() method refer ‘this’ to any object.

7. Describe Popular JavaScript Libraries/Frameworks

• React: A widely used JavaScript library for building user interfaces. It follows a component-based architecture and emphasizes reusability and efficiency.
• Angular: A comprehensive framework for building large-scale applications. It provides a complete solution for structuring, building, and deploying complex web applications.
• Vue.js: A progressive JavaScript framework for building user interfaces. It focuses on simplicity and adaptability, making it suitable for both small and large-scale projects.
• jQuery: A fast and feature-rich JavaScript library for DOM manipulation, event handling, animations, and AJAX. It simplifies common tasks and supports older browser versions.
• Express.js: A minimalist web application framework for Node.js. It provides a flexible set of features for building web APIs and server-side applications.
• D3.js: A powerful library for creating data visualizations on the web. It allows you to bind data to DOM elements and apply transformations to create dynamic and interactive visualizations.

8. What is Types of JavaScript Testing ?

• Unit Testing: Tests individual units of code, such as functions or classes, in isolation.
• Integration Testing: Tests the interaction between multiple units/modules to ensure they work correctly together.
• End-to-End (E2E) Testing: Tests the entire application flow, simulating real user interactions and validating the behavior of all components.

9. What is the difference between callbacks, promises, and async/await?

Callbacks are functions that are passed as arguments to other functions and executed once a particular task is completed.

Promises, on the other hand, provide a better way of handling asynchronous operations. They allow for better control flow and error handling.

Async/await, introduced in ES8, is a syntax sugar on top of promises, enabling asynchronous code to be written in a more synchronous-like manner.

10. What is the difference between null and undefined?

In JS, null and undefined are both used to represent the absence of a value, but they have different meanings.

null represents a deliberate non-value, often used to indicate that a variable intentionally has no value.

On the other hand, undefined represents a value that has not been initialized, often caused by a variable that has been declared but not assigned a value as in the previous answer.