5 Javascript Interview Questions and Answers for Beginners

Question: What is the difference between null and undefined in JavaScript?

Both null and undefined represent the absence of a value in JavaScript, but they are used in slightly different contexts.

null is an explicitly assigned value that represents the absence of any object value. It is a primitive value and must be assigned explicitly.

undefined represents the absence of a defined value. When a variable is declared but not initialized, it automatically gets the value undefined.

Explanation: For example, consider the following code:

Question: What is the significance of closures in JavaScript?Closures are an essential concept in JavaScript and allow functions to "remember" the environment in which they were created. A closure is created when an inner function references variables from its outer function even after the outer function has finished executing.

Explanation: Here's an example:

In this example, innerFunction forms a closure and retains access to outerVariable, even though outerFunction has already executed.

Question: What are the differences between let, const, and var for variable declaration?

These are three ways to declare variables in JavaScript, each with different scoping rules and mutability.

var: Variables declared with var are function-scoped and are hoisted to the top of their function or global scope. They can be redeclared and reassigned within their scope.

let: Variables declared with let are block-scoped and are not hoisted. They can be reassigned but not redeclared within their scope.

const: Variables declared with const are also block-scoped and not hoisted. They cannot be reassigned or redeclared once they are assigned a value. However, note that the contents of a const variable can be mutated if it holds a reference to a mutable object.

Explanation: Here's an example to illustrate the differences:

Question: How does prototypal inheritance work in JavaScript?JavaScript uses prototypal inheritance, where objects can inherit properties and methods from other objects. Each object in JavaScript has an internal link to another object called its prototype. If a property or method is not found on the object itself, JavaScript looks up the prototype chain until it finds the property or until the chain ends at the root object, Object.prototype.

Here's an example to illustrate prototypal inheritance:

In this example, both person1 and person2 have access to the greet method through the prototype chain.

Question: What are callbacks, and how are they used in JavaScript?

Answer: Callbacks are functions passed as arguments to other functions and are executed after the completion of the parent function. They are commonly used in asynchronous operations like reading files, making API calls, or handling events.

Here's a simple example using a callback with a setTimeout function:

The output will be:

In this example, farewell is the callback function, and it is executed after the greet function completes its task of greeting Alice.

Remember that while preparing for interviews, understanding the concepts and being able to explain your thought process is crucial. These questions are meant to test your understanding of JavaScript fundamentals, so practice explaining the concepts in a clear and concise manner. Good luck!

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