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Title
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Coding
System design
Quiz
JavaScript
React
105
quizzes
210
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1
Which of the following is NOT a JavaScript primitive data type?
JavaScript
Data Types
Easy
4 options
Which operator is used to check the data type of a variable in JavaScript?
JavaScript
Type Checking
Easy
4 options
Which statement about var, let, and const is correct?
JavaScript
Variables
Easy
4 options
Which is NOT a common way to convert a string to a number in JavaScript?
JavaScript
Conversion
Easy
4 options
How do you embed an expression inside a template literal?
JavaScript
Template Literals
Easy
4 options
What does the spread operator (...) do with an array like [...arr]?
JavaScript
Spread
Easy
4 options
What is hoisting in JavaScript?
JavaScript
Hoisting
Easy
4 options
How can you best avoid problems caused by hoisting?
JavaScript
Hoisting
Easy
4 options
What is the key difference between == and === in JavaScript?
JavaScript
Equality
Easy
4 options
What is the difference between break and continue?
JavaScript
Control Flow
Easy
4 options
What is the syntax of the ternary operator?
JavaScript
Operators
Easy
4 options
Which array method gives you both the element and its index in the callback?
JavaScript
Arrays
Easy
4 options
What does the switch statement do when no case matches?
JavaScript
Control Flow
Easy
4 options
What do rest parameters (...args) do?
JavaScript
Functions
Easy
4 options
What is the difference between a parameter and an argument?
JavaScript
Functions
Easy
4 options
What is a key benefit of arrow functions in callbacks?
JavaScript
Functions
Easy
4 options
What is a callback function?
JavaScript
Callbacks
Easy
4 options
What is a typical use case for anonymous functions?
JavaScript
Anonymous Functions
Easy
4 options
When is a default parameter's value used?
JavaScript
Default Parameters
Easy
4 options
Which is NOT a standard way to create an object in JavaScript?
JavaScript
Objects
Easy
4 options
Which array method transforms each element and returns a new array?
JavaScript
Arrays
Easy
4 options
What does destructuring assignment let you do?
JavaScript
Destructuring
Easy
4 options
How can you reliably check whether an object is empty?
JavaScript
Objects
Easy
4 options
What is the key difference between synchronous and asynchronous functions?
JavaScript
Async
Easy
4 options
What are the three states of a Promise?
JavaScript
Promises
Easy
4 options
Which keywords implement inheritance in ES2015 classes?
JavaScript
Classes
Easy
4 options
What is the convention when naming a constructor function?
JavaScript
Constructors
Easy
4 options
Which are the three main types of scope in JavaScript?
JavaScript
Scope
Easy
4 options
A common way to preserve the correct `this` when passing a method as a callback is:
JavaScript
This
Easy
4 options
What is the DOM?
JavaScript
Dom
Easy
4 options
Which method creates a new element to insert into the DOM?
JavaScript
Dom
Easy
4 options
How do you attach an event listener to an element?
JavaScript
Events
Easy
4 options
In event bubbling, an event propagates:
JavaScript
Events
Easy
4 options
How do you stop an event's default action, e.g. a form submitting?
JavaScript
Events
Easy
4 options
How do you toggle a CSS class on an element in JavaScript?
JavaScript
Dom
Easy
4 options
What is the difference between the Window and Document objects?
JavaScript
Bom
Easy
4 options
What does the Fetch API's fetch() function return?
JavaScript
Fetch
Easy
4 options
Which is a popular third-party library for making API calls?
JavaScript
Api Calls
Easy
4 options
Which HTML tag shows content when JavaScript is disabled?
JavaScript
Html Js
Easy
4 options
How do you redirect the browser to a new URL in JavaScript?
JavaScript
Navigation
Easy
4 options
Why are JavaScript modules useful?
JavaScript
Modules
Easy
4 options
How do you export a single default value from an ES module?
JavaScript
Modules
Easy
4 options
Where are a JavaScript module's metadata fields (name, version, dependencies) typically defined?
JavaScript
Modules
Easy
4 options
Which are the three main categories of JavaScript errors?
JavaScript
Errors
Easy
4 options
What is the role of a try...catch block?
JavaScript
Errors
Easy
4 options
What is the difference between a Set and a Map?
JavaScript
Collections
Easy
4 options
How do you convert a Set to an array?
JavaScript
Collections
Easy
4 options
What is lazy loading?
JavaScript
Performance
Easy
4 options
What is caching used for in front-end performance?
JavaScript
Performance
Easy
4 options
Which tool provides a Performance panel and audits for analyzing JS performance?
JavaScript
Performance
Easy
4 options
Which testing type checks individual components in isolation?
JavaScript
Testing
Easy
4 options
Which is a popular JavaScript testing framework known for integration with React?
JavaScript
Testing
Easy
4 options
What are design patterns?
JavaScript
Design Patterns
Easy
4 options
Why is input validation important for security?
JavaScript
Security
Easy
4 options
Which statement pauses execution to invoke the debugger where supported?
JavaScript
Debugging
Easy
4 options
What is the standard way to share code between JavaScript files?
JavaScript
Modules
Easy
4 options
What kind of technology is React?
React
React Basics
Easy
4 options
Which statement best describes a React Element?
React
React Elements
Easy
4 options
What does JSX stand for?
React
Jsx
Easy
4 options
What is the key difference between state and props?
React
State Props
Easy
4 options
What is the purpose of the `key` prop when rendering lists?
React
Keys
Easy
4 options
Using array indices as keys is problematic mainly when the list is:
React
Keys
Easy
4 options
In a controlled component, the source of truth for a form input's value is:
React
Controlled Components
Easy
4 options
Which is a common pitfall when using React Context?
React
Context
Easy
4 options
What do hooks let you do?
React
Hooks
Easy
4 options
Where must React hooks be called?
React
Hooks Rules
Easy
4 options
When does `useEffect` run relative to the browser paint?
React
Effects
Easy
4 options
What does the updater (callback) form `setCount(c => c + 1)` guarantee?
React
Setstate
Easy
4 options
What does an empty dependency array `[]` in useEffect mean?
React
Effects
Easy
4 options
What is `useRef` primarily used for?
React
Useref
Easy
4 options
What does `useCallback` return?
React
Usecallback
Easy
4 options
What is `useMemo` used for?
React
Usememo
Easy
4 options
What does `useReducer` return?
React
Usereducer
Easy
4 options
What does the `useId` hook generate?
React
Useid
Easy
4 options
What triggers a re-render in React?
React
Rendering
Easy
4 options
What are React Fragments used for?
React
Fragments
Easy
4 options
What was `forwardRef()` historically used for?
React
Refs
Easy
4 options
What is the most idiomatic way to fully reset a component's state?
React
State Reset
Easy
4 options
Why does React recommend against mutating state directly?
React
Immutability
Easy
4 options
What do error boundaries do?
React
Error Boundaries
Easy
4 options
Which library encourages testing React components the way users interact with them?
React
Testing
Easy
4 options
What is React hydration?
React
Hydration
Easy
4 options
What do React Portals let you do?
React
Portals
Easy
4 options
Which browser extension helps inspect the React component tree?
React
Debugging
Easy
4 options
What does React Strict Mode render to the page?
React
Strict Mode
Easy
4 options
Which libraries are commonly used to localize React applications?
React
I18n
Easy
4 options
What is code splitting in a React application?
React
Code Splitting
Easy
4 options
In 2026, what should you adopt first to reduce context-driven re-renders?
React
Context Perf
Easy
4 options
What is a higher order component (HOC)?
React
Hoc
Easy
4 options
What data flow does the Flux pattern enforce?
React
Flux
Easy
4 options
In React's one-way data flow, data moves:
React
Data Flow
Easy
4 options
What does the React documentation recommend instead of hand-rolling useEffect + fetch?
React
Async Data
Easy
4 options
What is server-side rendering (SSR) in React?
React
Ssr
Easy
4 options
When does static generation (SSG) render pages?
React
Ssg
Easy
4 options
In the presentational vs container pattern, presentational components:
React
Patterns
Easy
4 options
Which is a common data-fetching pitfall in React?
React
Data Fetching Pitfalls
Easy
4 options
What is a render prop?
React
Render Props
Easy
4 options
Which is a React anti-pattern?
React
Anti Patterns
Easy
4 options
Which tool fits purely local component state?
React
State Decision
Easy
4 options
What is the composition pattern in React?
React
Composition
Easy
4 options
What is the 'virtual DOM' in React?
React
Virtual Dom
Easy
4 options
What is React Fiber?
React
Fiber
Easy
4 options
What is reconciliation in React?
React
Reconciliation
Easy
4 options
What does React Suspense let you do?
React
Suspense
Easy
4 options
What happens immediately when a useState setter is called?
React
Setstate
Easy
4 options
