State re-render behavior in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to update state and trigger a re-render in a React functional component.

React

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

function increment() {
  setCount([1]);
}

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

Acount + 1

Bcount - 1

C0

Dcount

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same value without change, so no re-render happens.

Decreasing the count instead of increasing it.

Using setCount(count + 1) updates the state by increasing the count by 1, which triggers a re-render.

2fill in blank

medium

Complete the code to correctly read the current state value inside a functional component.

React

const [name, setName] = useState('');

return <p>Hello, [1]!</p>;

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

AName

Bname

CsetName

DuseState

Attempts:

3 left

💡 Hint

Common Mistakes

Using the setter function instead of the state value.

Using a capitalized variable that does not exist.

The variable name holds the current state value, so it should be used to display the name.

3fill in blank

hard

Fix the error in the code to properly update state based on the previous state value.

React

setCount([1] => [1] + 1);

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

Aprev

Bcount

Cstate

Dvalue

Attempts:

3 left

💡 Hint

Common Mistakes

Using the state variable directly inside the updater function.

Using a variable name that is not defined.

Using a function with a parameter like prev ensures you get the latest state value when updating.

4fill in blank

hard

Fill both blanks to create a state object and update one property without losing others.

React

const [user, setUser] = useState({ name: '', age: 0 });

setUser({ ...user, [1]: [2] });

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

Aname

Bage

C'John'

D30

Attempts:

3 left

💡 Hint

Common Mistakes

Not using the spread operator, which overwrites the whole object.

Mixing property names and values incorrectly.

Using the spread operator ...user keeps existing properties, and updating name to 'John' changes only that property.

5fill in blank

hard

Fill all three blanks to create a memoized callback that updates state only when dependencies change.

React

const increment = useCallback(() => {
  setCount([1] + 1);
}, [[2], [3]]);

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

Acount

CsetCount

Dincrement

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving dependencies empty or missing required variables.

Using incorrect variable names in the dependencies.

The callback uses the current count to update state, and the dependencies include count and setCount to ensure the function updates correctly.

Practice

1. What happens in React when you update a component's state using setState?

easy

A. React re-renders that component and its children to update the UI.

B. React reloads the entire page to reflect the changes.

C. React ignores the change unless the component is a class component.

D. React only updates the state internally without changing the UI.

Solution

1. Step 1: Understand what setState does

   Using setState tells React that the component's state has changed and it needs to update the UI.

2. Step 2: Know React's rendering behavior

   React re-renders the component where state changed and all its child components to reflect the new state.

3. Final Answer:

   React re-renders that component and its children to update the UI. -> Option A

4. Quick Check:

   State change triggers re-render = B [OK]

Hint: State change triggers re-render of component and children [OK]

Common Mistakes:

• Thinking React reloads the whole page
• Believing state changes don't affect functional components
• Assuming UI doesn't update automatically

2. Which of the following is the correct way to update state in a React functional component?

easy

A. this.state.count = 1;

B. updateState({count: 1});

C. count = 1;

D. setCount(1);

Solution

1. Step 1: Recall how state is updated in functional components

   Functional components use the useState hook, which returns a setter function like setCount.

2. Step 2: Identify the correct syntax

   Calling setCount(1) updates the state and triggers a re-render.

3. Final Answer:

   setCount(1); -> Option D

4. Quick Check:

   Use setter function from useState = D [OK]

Hint: Use the setter function from useState to update state [OK]

Common Mistakes:

• Trying to assign state directly like a variable
• Using this.state in functional components
• Using non-existent functions like updateState

3. Consider this React component code:

function Counter() {
  const [count, setCount] = React.useState(0);
  React.useEffect(() => {
    console.log('Rendered with count:', count);
  });
  return  setCount(count + 1)}>Click {count};
}

What will be logged to the console after clicking the button twice?

medium

A. 'Rendered with count: 1' then 'Rendered with count: 2'

B. 'Rendered with count: 0' only once

C. 'Rendered with count: 2' only once

D. No logs will appear

Solution

1. Step 1: Understand useEffect without dependencies

   Without a dependency array, useEffect runs after every render.

2. Step 2: Trace state changes on clicks

   Initial count is 0. Clicking once sets count to 1, triggering a render and logging 'Rendered with count: 1'. Clicking again sets count to 2, triggering another render and logging 'Rendered with count: 2'.

3. Final Answer:

   'Rendered with count: 1' then 'Rendered with count: 2' -> Option A

4. Quick Check:

   State updates cause re-render and useEffect logs each time = A [OK]

Hint: useEffect without deps runs after every render [OK]

Common Mistakes:

• Thinking useEffect runs only once
• Assuming logs show initial state only
• Ignoring that state updates cause re-renders

4. What is wrong with this React code snippet?

function Example() {
  const [value, setValue] = React.useState(0);
  function update() {
    value = value + 1;
  }
  return Increase;
}

medium

A. useState hook is used incorrectly; it needs two arguments.

B. Directly assigning to state variable does not update state or trigger re-render.

C. The function update should be async to update state.

D. The button should use onChange instead of onClick.

Solution

1. Step 1: Identify how state is updated

   State variables from useState are read-only. Direct assignment like value = value + 1 does not update React state.

2. Step 2: Understand correct state update method

   State must be updated using the setter function setValue to trigger re-render.

3. Final Answer:

   Directly assigning to state variable does not update state or trigger re-render. -> Option B

4. Quick Check:

   State updates require setter function = C [OK]

Hint: Never assign state variable directly; use setter function [OK]

Common Mistakes:

• Assigning state variable directly
• Confusing event handlers like onChange vs onClick
• Thinking useState needs two arguments

5. You have a parent component that renders a child component. The parent updates its state with setParentState. Which components re-render when the parent's state changes?

hard

A. Only the child component re-renders; the parent does not.

B. Only the parent component re-renders; the child does not.

C. Both the parent and child components re-render.

D. Neither component re-renders automatically.

Solution

1. Step 1: Understand React's re-render on state change

   When a parent component's state changes, React re-renders that parent component.

2. Step 2: Know child component rendering behavior

   Since the child is rendered inside the parent, React also re-renders the child unless memoization or other optimizations are used.

3. Final Answer:

   Both the parent and child components re-render. -> Option C

4. Quick Check:

   Parent state change re-renders parent and children = A [OK]

Hint: Parent state change re-renders parent and children by default [OK]

Common Mistakes:

• Thinking child won't re-render automatically
• Assuming only child re-renders
• Believing no re-render happens without explicit calls