React context in client components in NextJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this React context usage in a client component?

Consider this Next.js client component using React context. What will be rendered inside the <p> tag?

NextJS

'use client';

import React, { createContext, useContext } from 'react';

const ThemeContext = createContext('light');


function DisplayTheme() {
  const theme = useContext(ThemeContext);
  return <p>{`Current theme is: ${theme}`}</p>;
}

export default function Page() {
  return (
    <ThemeContext.Provider value="dark">
      <DisplayTheme />
    </ThemeContext.Provider>
  );
}

A Current theme is: dark

B Current theme is: undefined

C Error: useContext can only be used inside a Provider

D Current theme is: light

💡 Hint

Remember that the value passed to the Provider overrides the default context value.

The ThemeContext.Provider wraps DisplayTheme and provides the value "dark". So useContext(ThemeContext) returns "dark". The default value "light" is ignored because the Provider is present.

❓ lifecycle

intermediate

When does a React client component re-render when context changes?

Given a React client component consuming context, when will it re-render?

NextJS

'use client';

import React, { createContext, useContext, useState } from 'react';

const CountContext = createContext(0);


function ShowCount() {
  const count = useContext(CountContext);
  return <p>{`Count is ${count}`}</p>;
}

export default function Counter() {
  const [count, setCount] = useState(0);

  return (
    <CountContext.Provider value={count}>
      <ShowCount />
      <button onClick={() => setCount(count + 1)}>Increment</button>
    </CountContext.Provider>
  );
}

A ShowCount re-renders only when the button is clicked twice

B ShowCount never re-renders because context value is primitive

C ShowCount re-renders every time the count state changes

D ShowCount re-renders only on initial render

💡 Hint

Think about how React context triggers updates when the Provider's value changes.

When the count state changes, the CountContext.Provider value changes, causing all consumers like ShowCount to re-render with the new value.

🔧 Debug

advanced

Why does this React client component throw an error using context?

Examine this code snippet. Why does it throw an error when rendering?

NextJS

'use client';

import React, { createContext, useContext } from 'react';

const UserContext = createContext(null);

function UserName() {
  const user = useContext(UserContext);
  return <p>{user.name}</p>;
}

export default function Page() {
  return <UserName />;
}

A Error because UserName uses useContext without a Provider wrapping it

B Error because createContext was called with null instead of an object

C No error, renders empty paragraph

D Error because useContext must be called inside a useEffect hook

💡 Hint

Check if the component consuming context is wrapped in a Provider.

The UserName component calls useContext(UserContext) but no UserContext.Provider is wrapping it, so the context value is null. Accessing user.name causes a runtime error.

📝 Syntax

advanced

Which option correctly defines a React context and uses it in a client component?

Choose the option that correctly creates a React context and consumes it in a client component without syntax errors.

A

import React, { createContext, useContext } from 'react';

const MyContext = createContext('default');

'use client';

function Comp() {
  const val = useContext(MyContext);
  return <div>{val}</div>;
}

export default function Page() {
  return <MyContext.Provider value="hello"><Comp /></MyContext.Provider>;
}

B

import React, { createContext, useContext } from 'react'

const MyContext = createContext('default')

function Comp() {
  const val = useContext(MyContext)
  return <div>{val}</div>
}

export default function Page() {
  return <MyContext.Provider value="hello"><Comp /></MyContext.Provider>
}

C

import React, { createContext, useContext } from 'react';

const MyContext = createContext('default');

function Comp() {
  const val = useContext(MyContext);
  return <div>{val}</div>;
}

export default function Page() {
  return <MyContext.Provider value="hello"><Comp /></MyContext.Provider>;
}

D

'use client';

import React, { createContext, useContext } from 'react';

const MyContext = createContext('default');


function Comp() {
  const val = useContext(MyContext);
  return <div>{val}</div>;
}

export default function Page() {
  return <MyContext.Provider value="hello"><Comp /></MyContext.Provider>;
}

💡 Hint

Remember to mark client components with 'use client'; directive.

Option D correctly includes the 'use client'; directive at the top of the client component file, uses semicolons, and properly wraps Comp with the Provider. Option D is missing the directive at the top of the file (it is after imports). Option D misses semicolons and the directive. Option D misses the directive entirely.

❓ state_output

expert

What is the final rendered output after clicking the button twice in this Next.js client component using context?

Analyze the code below. After clicking the "Add" button twice, what will be the rendered output inside the <div>?

NextJS

'use client';

import React, { createContext, useContext, useState, useCallback } from 'react';

const NumberContext = createContext(0);


function DisplayNumber() {
  const number = useContext(NumberContext);
  return <div>{`Number: ${number}`}</div>;
}

export default function Counter() {
  const [num, setNum] = useState(0);

  const add = useCallback(() => {
    setNum(n => n + 1);
  }, []);

  return (
    <NumberContext.Provider value={num}>
      <DisplayNumber />
      <button onClick={add}>Add</button>
    </NumberContext.Provider>
  );
}

A Number: 0

B Number: 2

C Number: 1

D Number: undefined

💡 Hint

Consider how state updates and context value changes affect re-rendering.

Each click increments num by 1. After two clicks, num is 2. The context value updates to 2, so DisplayNumber renders "Number: 2".

Practice

1. What is the main purpose of React context in client components?

easy

A. To share data easily without passing props through every component

B. To create new HTML elements dynamically

C. To style components with CSS

D. To handle browser events like clicks and scrolls

Solution

1. Step 1: Understand React context role

   React context allows sharing data across components without manually passing props at every level.

2. Step 2: Compare with other options

   Creating HTML elements, styling, and event handling are unrelated to React context's main purpose.

3. Final Answer:

   To share data easily without passing props through every component -> Option A

4. Quick Check:

   React context = data sharing without prop drilling [OK]

Hint: Context shares data without passing props everywhere [OK]

Common Mistakes:

• Confusing context with styling or event handling
• Thinking context creates HTML elements
• Believing context is for CSS only

2. Which of the following is the correct way to access React context in a client component?

easy

A. const value = useRef(MyContext);

B. const value = useState(MyContext);

C. const value = useContext(MyContext);

D. const value = useEffect(MyContext);

Solution

1. Step 1: Identify the hook for context access

   React provides the useContext hook to access context values inside client components.

2. Step 2: Eliminate other hooks

   useState manages state, useEffect handles side effects, and useRef manages references, none access context.

3. Final Answer:

   const value = useContext(MyContext); -> Option C

4. Quick Check:

   useContext hook = context access [OK]

Hint: Use useContext to read context in components [OK]

Common Mistakes:

• Using useState or useEffect instead of useContext
• Trying to access context without a hook
• Confusing useRef with context access

3. Given this code snippet in a Next.js client component:

import { createContext, useContext } from 'react';

const ThemeContext = createContext('light');

export default function DisplayTheme() {
  const theme = useContext(ThemeContext);
  return <div>Current theme: {theme}</div>;
}

What will be displayed if DisplayTheme is rendered without a ThemeContext.Provider?

medium

A. Current theme: undefined

B. Current theme: light

C. Current theme: null

D. Error: Context provider missing

Solution

1. Step 1: Check default context value

   The context is created with default value 'light', so if no Provider wraps the component, this default is used.

2. Step 2: Understand useContext behavior without Provider

   useContext returns the default value when no Provider is found above in the tree.

3. Final Answer:

   Current theme: light -> Option B

4. Quick Check:

   Default context value used without Provider [OK]

Hint: Default context value shows if no Provider wraps component [OK]

Common Mistakes:

• Expecting undefined or null instead of default value
• Thinking it throws an error without Provider
• Confusing default value with null

4. Identify the error in this Next.js client component using React context:

import { createContext, useContext } from 'react';

const UserContext = createContext();

export default function UserName() {
  const user = useContext(UserContext);
  return <div>User: {user.name}</div>;
}

medium

A. UserContext is not wrapped with a Provider, so user is undefined causing error

B. useContext hook is used incorrectly; should pass a string

C. Missing import of React from 'react'

D. Cannot use context inside client components

Solution

1. Step 1: Check context default value and Provider usage

   UserContext is created without a default value and no Provider is shown wrapping UserName, so useContext returns undefined.

2. Step 2: Understand error cause

   Accessing user.name when user is undefined causes a runtime error.

3. Final Answer:

   UserContext is not wrapped with a Provider, so user is undefined causing error -> Option A

4. Quick Check:

   Undefined context value causes error accessing properties [OK]

Hint: Always wrap context consumers with Provider to avoid undefined errors [OK]

Common Mistakes:

• Assuming useContext needs a string argument
• Forgetting to wrap with Provider
• Thinking React import is mandatory in Next.js 13+ client components

5. You want to create a theme toggle in a Next.js client component using React context. Which approach correctly updates the theme value across components?

hard

A. Pass theme as a prop to every component without using context

B. Create ThemeContext with a fixed string; update theme by changing the string directly in components

C. Use useState inside each component separately without context to manage theme

D. Create a ThemeContext with state and provide toggle function via Provider; use useContext to read and update theme

Solution

1. Step 1: Understand state sharing with context

   To update theme globally, context must provide state and a function to toggle it, so all components share and update the same state.

2. Step 2: Evaluate other options

   Fixed string cannot update dynamically; separate useState per component does not share state; passing props everywhere defeats context purpose.

3. Final Answer:

   Create a ThemeContext with state and provide toggle function via Provider; use useContext to read and update theme -> Option D

4. Quick Check:

   Context with state and updater shares dynamic data [OK]

Hint: Use context Provider with state and updater for shared dynamic data [OK]

Common Mistakes:

• Trying to update fixed context value directly
• Using separate state in each component without sharing
• Passing props everywhere instead of using context