Top 15+ JavaScript Interview Questions and Answers [PDF]

Qus-1: What is JavaScript?

JavaScript is a high-level, interpreted programming language used for web development. It runs on the client side and server-side, and can be used to create interactive and dynamic web pages.

Qus-2: What are the data types in JavaScript?

The data types in JavaScript are: string, number, boolean, null, undefined, object, and symbol.

Qus-3: What is the difference between == and === in JavaScript?

The double equal sign (==) in JavaScript is a loose comparison operator that only compares the value, while the triple equal sign (===) is a strict comparison operator that compares both the value and the type.

Qus-4: What is the difference between the null and undefined values in JavaScript?

The null value represents an intentional absence of any object value, while the undefined value represents a variable that has not been assigned a value.

Qus-5: What are the different types of operators in JavaScript?What are the different types of operators in JavaScript?

The different types of operators in JavaScript are: arithmetic operators, comparison operators, logical operators, assignment operators, and bitwise operators.

Qus-6: What is the use of 'typeof' operator in JavaScript?

The typeof operator in JavaScript is used to return the data type of an operand.

Qus-7: How do you declare variables in JavaScript?

You can declare variables in JavaScript using the var, let, or const keywords.

Qus-8: What is the difference between let, var, and const in JavaScript?

The main difference between let, var, and const in JavaScript is their scope and re-assignment ability. var is function scoped, let and const are block-scoped, and const cannot be reassigned once defined.

Qus-9: How do you create an object in JavaScript?

You can create an object in JavaScript using object literal syntax, or by using the Object constructor.

Qus-10: How do you declare and define a function in JavaScript?

You can declare a function in JavaScript using the function keyword, followed by the function name, and the parameter list. The function definition is then enclosed in curly braces {}.

Qus-11: What is a callback function in JavaScript?

A callback function in JavaScript is a function that is passed as an argument to another function and is executed inside that function.

Qus-12: What is an arrow function in JavaScript?

An arrow function in JavaScript is a shorthand way of declaring a function. It uses the "=>" syntax and does not have its own 'this' keyword.

Qus-13: What is a closure in JavaScript?

A closure in JavaScript is a function that has access to variables in its outer lexical scope, even after that scope has closed.

Qus-14: How do you inherit from a class in JavaScript?

You can inherit from a class in JavaScript by using the 'extends' keyword and the 'super' keyword.

Qus-15: What is the prototype in JavaScript?

The prototype in JavaScript is a property of objects that allows for inheritance and the sharing of properties and methods between objects.

Qus-16: What is an event in JavaScript?

An event in JavaScript is an action or occurrence that happens in the browser, such as a click, a page load, or a key press.

Qus-17: How do you handle errors in JavaScript?

You can handle errors in JavaScript using try-catch blocks, which allow you to catch and handle errors that occur in your code.

Qus-18: What is the difference between synchronous and asynchronous code in JavaScript?

Synchronous code in JavaScript is executed in a single thread, meaning that each line of code is executed one after the other in a sequence. When a function is called in synchronous code, the code execution is paused until the function returns a value.

Asynchronous code, on the other hand, allows code to run in parallel with other code, meaning that multiple functions can run at the same time. When an asynchronous function is called, the code execution continues without waiting for the function to complete. Instead, the function is executed in the background and once it's finished, a callback function is called to handle the result.

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