Sharing state between components in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a state variable using React hooks.

React

const [count, setCount] = [1](0);

Drag options to blanks, or click blank then click option'

AuseEffect

BuseState

CuseContext

DuseRef

Attempts:

3 left

💡 Hint

Common Mistakes

Using useEffect instead of useState to create state.

Trying to use useContext without creating a context first.

The useState hook creates a state variable and a function to update it.

2fill in blank

medium

Complete the code to pass state as a prop from a parent to a child component.

React

<ChildComponent [1]={count} />

Drag options to blanks, or click blank then click option'

Adata

Bvalue

Cstate

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using a generic prop name like 'value' when the child expects 'count'.

Forgetting to pass the state as a prop.

Props are named attributes. Here, the prop name is count to pass the state value.

3fill in blank

hard

Fix the error in the child component to correctly receive props.

React

function ChildComponent([1]) {
  return <p>{count}</p>;
}

Drag options to blanks, or click blank then click option'

A{count}

Bprops

Ccount

Dstate

Attempts:

3 left

💡 Hint

Common Mistakes

Using props which would require props.count inside the component.

Using curly braces inside the function parameter incorrectly.

To correctly access the count prop in the child component, destructure it from the props object using ({count}) in the function parameters.

4fill in blank

hard

Fill both blanks to create a context and provide it to child components.

React

const MyContext = React.[1]();

function App() {
  const [value, setValue] = React.useState(0);
  return (
    <MyContext.[2] value={{ count: value }}>
      <ChildComponent />
    </MyContext.[2]>
  );
}

Drag options to blanks, or click blank then click option'

AcreateContext

BuseContext

CProvider

DConsumer

Attempts:

3 left

💡 Hint

Common Mistakes

Using useContext instead of createContext to create the context.

Using Consumer instead of Provider to supply the value.

createContext creates a context object. The Provider component supplies the context value to children.

5fill in blank

hard

Fill all three blanks to consume context value inside a child component.

React

function ChildComponent() {
  const value = React.[1](MyContext);
  return <p>Value: {value.[2]</p>;
}

// Context value is an object like { count: 5 }

// Access the count property with [3]

Drag options to blanks, or click blank then click option'

AuseContext

Bcount

Cvalue.count

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to use useState or other hooks instead of useContext.

Accessing the whole value object instead of a specific property.

useContext hook reads the context value. The value is an object, so access the count property to display it.

Practice

1. What is the main reason to share state between React components?

easy

A. To keep data consistent and allow multiple components to use the same information

B. To make each component have its own separate data

C. To avoid using props in components

D. To prevent components from rendering

Solution

1. Step 1: Understand the purpose of shared state

   Sharing state means keeping data in one place so multiple components can access and update it.

2. Step 2: Identify the benefit of shared state

   This helps keep the app consistent and easier to manage because all components see the same data.

3. Final Answer:

   To keep data consistent and allow multiple components to use the same information -> Option A

4. Quick Check:

   Sharing state = data consistency [OK]

Hint: Shared state means one source of truth for data [OK]

Common Mistakes:

• Thinking each component should have its own separate state always
• Confusing props with state sharing
• Believing sharing state prevents rendering

2. Which of the following is the correct way to pass state and its updater from a parent to a child component in React?

easy

A. <Child state=setState />

B. <Child state={state} setState={setState} />

C. <Child>{setState}</Child>

D. <Child state>{state}</Child>

Solution

1. Step 1: Recall how props are passed in JSX

   Props are passed as attributes with values, like propName={value}.

2. Step 2: Identify correct syntax for passing state and updater

   The parent passes both state and setState as props: <Child state={state} setState={setState} />.

3. Final Answer:

   <Child state={state} setState={setState} /> -> Option B

4. Quick Check:

   Props syntax = attribute=value [OK]

Hint: Props use attribute={value} syntax [OK]

Common Mistakes:

• Using children tags incorrectly to pass props
• Passing updater as a child instead of prop
• Missing curly braces around JavaScript expressions

3. Given this React code, what will be displayed when the button is clicked?

function Parent() {
  const [count, setCount] = React.useState(0);
  return <Child count={count} setCount={setCount} />;
}

function Child({ count, setCount }) {
  return <>
    <p>Count: {count}</p>
    <button onClick={() => setCount(count + 1)}>Increment</button>
  </>;
}

medium

A. Count: 0 initially, but button does nothing

B. Count: 1 initially, then stays the same on click

C. Count: 0 initially, then increases by 1 each click

D. Error because setCount is not defined

Solution

1. Step 1: Understand initial state and props

   The parent sets count to 0 initially and passes count and setCount to Child.

2. Step 2: Analyze button click behavior

   Clicking the button calls setCount(count + 1), increasing count by 1 each time.

3. Final Answer:

   Count: 0 initially, then increases by 1 each click -> Option C

4. Quick Check:

   useState count increments on click [OK]

Hint: State updates cause re-render with new value [OK]

Common Mistakes:

• Thinking count starts at 1
• Assuming button does not update state
• Confusing setCount usage or scope

4. Identify the error in this React code that tries to share state between components:

function Parent() {
  const [value, setValue] = React.useState('');
  return <Child value={value} setValue={value} />;
}

function Child({ value, setValue }) {
  return <input value={value} onChange={e => setValue(e.target.value)} />;
}

medium

A. value state is not initialized properly

B. onChange handler syntax is invalid

C. Child component should not receive props

D. setValue prop is passed incorrectly as value instead of the updater function

Solution

1. Step 1: Check props passed from Parent

   Parent passes setValue={value} instead of setValue={setValue}, so setValue is a string, not a function.

2. Step 2: Understand effect on Child component

   Child tries to call setValue as a function on input change, but it is a string, causing an error.

3. Final Answer:

   setValue prop is passed incorrectly as value instead of the updater function -> Option D

4. Quick Check:

   Updater function must be passed, not state value [OK]

Hint: Pass updater function, not state value, as setState prop [OK]

Common Mistakes:

• Passing state value instead of updater function
• Ignoring prop names mismatch
• Assuming onChange syntax is wrong

5. You want to share a counter state between two sibling components in React. Which approach correctly shares the state so both siblings update together?

function Parent() {
  const [count, setCount] = React.useState(0);
  return (
    <>
      <SiblingA count={count} setCount={setCount} />
      <SiblingB count={count} setCount={setCount} />
    </>
  );
}

What is the key reason this works?

hard

A. State is lifted to the common parent and passed down, so both siblings share the same source

B. Each sibling has its own state, so they update independently

C. Using context API is required for siblings to share state

D. State is stored inside each sibling, but synchronized automatically

Solution

1. Step 1: Understand state lifting

   The parent holds the state and updater, passing them as props to both siblings.

2. Step 2: Explain shared state effect

   Because both siblings receive the same state and updater, changes in one update the parent's state, causing both to re-render with the new value.

3. Final Answer:

   State is lifted to the common parent and passed down, so both siblings share the same source -> Option A

4. Quick Check:

   Lift state up for shared data [OK]

Hint: Lift state to parent to share between siblings [OK]

Common Mistakes:

• Thinking siblings can share state without lifting or context
• Believing context API is always required
• Assuming state sync happens automatically without shared source