# JavaScript Essentials (React / Next.js)

> Variables (let/const/var), types, closures, this, arrow functions, destructuring, spread/rest

- 25 interview questions
- Junior
- [Interview Questions: React / Next.js](https://sharpskill.dev/en/technologies/react-next/interview-questions.md)

## 1. What is the main difference between let and var?

**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.

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

**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.

## 3. Which JavaScript data type is not primitive?

**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.

## 22 more questions available

- What does typeof null return in JavaScript?
- What is the difference between == and ===?

Sign up for free: https://sharpskill.dev/en/login

## Other React / Next.js interview topics

- [React Fundamentals](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-fundamentals.md): 20 questions, Junior
- [React Hooks](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-hooks.md): 22 questions, Junior
- [Component Lifecycle](https://sharpskill.dev/en/technologies/react-next/interview-questions/component-lifecycle.md): 18 questions, Junior
- [React Router](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-router.md): 20 questions, Junior
- [State Management with Context](https://sharpskill.dev/en/technologies/react-next/interview-questions/state-management-context.md): 18 questions, Junior
- [Forms & Controlled Components](https://sharpskill.dev/en/technologies/react-next/interview-questions/forms-controlled-components.md): 20 questions, Mid-Level
- [Data Fetching & API](https://sharpskill.dev/en/technologies/react-next/interview-questions/fetching-data-api.md): 20 questions, Mid-Level
- [React Query (TanStack Query)](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-query-tanstack.md): 20 questions, Mid-Level
- [Styling & CSS-in-JS](https://sharpskill.dev/en/technologies/react-next/interview-questions/styling-css-in-js.md): 18 questions, Mid-Level
- [Next.js Fundamentals](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-fundamentals.md): 25 questions, Mid-Level
- [TypeScript with React](https://sharpskill.dev/en/technologies/react-next/interview-questions/typescript-react.md): 20 questions, Mid-Level
- [Next.js Data Fetching](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-data-fetching.md): 24 questions, Mid-Level
- [Next.js Server Actions](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-server-actions.md): 20 questions, Mid-Level
- [Next.js Routing & Navigation](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-routing-navigation.md): 22 questions, Mid-Level
- [Next.js API Routes](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-api-routes.md): 20 questions, Mid-Level
- [Next.js Metadata & SEO](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-metadata-seo.md): 18 questions, Mid-Level
- [Next.js Middleware & Auth](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-middleware-auth.md): 22 questions, Mid-Level
- [React Testing](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-testing.md): 20 questions, Mid-Level
- [Zustand State Management](https://sharpskill.dev/en/technologies/react-next/interview-questions/zustand-state-management.md): 18 questions, Mid-Level
- [React Performance Optimization](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-performance-optimization.md): 22 questions, Senior
- [Error Boundaries & Error Handling](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-error-boundaries.md): 18 questions, Senior
- [Advanced React Patterns](https://sharpskill.dev/en/technologies/react-next/interview-questions/advanced-react-patterns.md): 20 questions, Senior
- [Next.js Advanced Features](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-advanced-features.md): 24 questions, Senior
- [Next.js Deployment & Production](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-deployment-production.md): 20 questions, Senior
- [Architecture & Design Patterns](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-architecture-patterns.md): 22 questions, Senior
- [React Server Components](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-server-components.md): 26 questions, Senior
- [Next.js Internationalization](https://sharpskill.dev/en/technologies/react-next/interview-questions/nextjs-internationalization.md): 20 questions, Senior
- [React Security & Best Practices](https://sharpskill.dev/en/technologies/react-next/interview-questions/react-security-best-practices.md): 22 questions, Senior

---

Source: SharpSkill (https://sharpskill.dev), tech interview preparation for your real stack.
HTML version of this page: https://sharpskill.dev/en/technologies/react-next/interview-questions/javascript-essentials