Question 1

What is the main difference between let and var?

Accepted Answer

let declares a block-scoped variable, meaning it only exists within the block where it's defined (between braces). var has function scope, which can cause unexpected bugs as the variable is accessible throughout the entire function. Using let by default prevents hoisting issues and variable leaks in loops or conditionals.

Question 2

Which declaration to use for a value that won't change?

Accepted Answer

const allows declaring a constant whose reference cannot be reassigned. This improves code readability by clearly indicating intent and prevents accidental modifications. Note that for objects and arrays, const only prevents reassigning the reference, not modifying the content. It's the best practice for declaring immutable values.

Question 3

Which JavaScript data type is not primitive?

Accepted Answer

Objects are the only non-primitive type in JavaScript. Primitive types include string, number, boolean, null, undefined, symbol and bigint. Objects are stored by reference in memory, while primitives are stored by value. This means modifying an object affects all references to it, whereas copying a primitive creates an independent new value.

JavaScript Essentials

What is the main difference between let and var?

Answer

Which declaration to use for a value that won't change?

Answer

Which JavaScript data type is not primitive?

Answer

What does typeof null return in JavaScript?

What is the difference between == and ===?

Other React / Next.js interview topics

React Fundamentals

React Hooks

Component Lifecycle

React Router

State Management with Context

Forms & Controlled Components

Data Fetching & API

React Query (TanStack Query)

Styling & CSS-in-JS

Next.js Fundamentals

TypeScript with React

Next.js Data Fetching

Next.js Server Actions

Next.js Routing & Navigation

Next.js API Routes

Next.js Metadata & SEO

Next.js Middleware & Auth

React Testing

Zustand State Management

React Performance Optimization

Error Boundaries & Error Handling

Advanced React Patterns

Next.js Advanced Features

Next.js Deployment & Production

Architecture & Design Patterns

React Server Components

Next.js Internationalization

React Security & Best Practices

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