Reusing logic with hooks in React - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom hook in React?

A custom hook is a JavaScript function whose name starts with 'use' and that can call other hooks. It lets you reuse stateful logic between components without changing their structure.

beginner

Why should you use custom hooks instead of duplicating code?

Custom hooks help keep code DRY (Don't Repeat Yourself). They make your code easier to read, maintain, and test by sharing logic in one place instead of copying it in many components.

intermediate

How do you create a custom hook to fetch data from an API?

You create a function starting with 'use', use React hooks like useState and useEffect inside it to manage data and loading state, then return the data and status so components can use it.

intermediate

Can custom hooks use other hooks inside them?

Yes! Custom hooks can call built-in hooks like useState, useEffect, or even other custom hooks. This lets you build complex reusable logic by combining simpler hooks.

beginner

What rules must you follow when writing custom hooks?

You must start the function name with 'use'. Inside, call hooks only at the top level and not inside loops or conditions. This ensures React can track hook calls correctly.

What prefix must a function have to be recognized as a custom hook in React?

A handle

B get

C use

D fetch

React recognizes custom hooks by the 'use' prefix, which signals that the function uses React hooks internally.

Which React hook is commonly used inside custom hooks to manage side effects like data fetching?

A useEffect

B useState

C useContext

D useRef

useEffect lets you perform side effects such as data fetching inside custom hooks.

Why should hooks be called only at the top level of a custom hook?

A To avoid infinite loops

B So React can track hook calls in the same order on every render

C To keep code clean

D To improve performance

React relies on hooks being called in the same order each render to manage state correctly.

What is a benefit of using custom hooks?

A They allow reusing logic without changing component structure

B They replace components

C They improve CSS styling

D They make components class-based

Custom hooks let you share logic between components without changing how components are built.

Can a custom hook return multiple values?

A No, only one value

B Only strings

C Only numbers

D Yes, usually as an object or array

Custom hooks often return multiple values bundled in an object or array for flexibility.

Explain how you would create and use a custom hook to share logic between two React components.

Think about how to wrap state and effects in a reusable function.

What are the rules you must follow when writing custom hooks in React and why are they important?

Consider React's hook call order and naming conventions.

Practice

1. What is the main purpose of creating a custom hook in React?

easy

A. To create new HTML elements

B. To reuse stateful logic across multiple components

C. To style components with CSS

D. To replace React components entirely

Solution

1. Step 1: Understand what hooks do

   Hooks let you use React features like state and effects inside functions.

2. Step 2: Identify the role of custom hooks

   Custom hooks let you reuse this logic in many components without repeating code.

3. Final Answer:

   To reuse stateful logic across multiple components -> Option B

4. Quick Check:

   Custom hooks = reuse logic [OK]

Hint: Custom hooks start with 'use' and share logic [OK]

Common Mistakes:

• Thinking hooks create UI elements
• Confusing hooks with styling tools
• Believing hooks replace components

2. Which of the following is the correct way to define a custom hook in React?

easy

A. function useCounter() { const [count, setCount] = useState(0); return [count, setCount]; }

B. function counterHook() { const [count, setCount] = useState(0); return [count, setCount]; }

C. function UseCounter() { const [count, setCount] = useState(0); return [count, setCount]; }

D. function counter() { const [count, setCount] = useState(0); return [count, setCount]; }

Solution

1. Step 1: Recall naming rules for custom hooks

   Custom hooks must start with 'use' to follow React conventions and enable linting.

2. Step 2: Check function names

   Only function useCounter() { const [count, setCount] = useState(0); return [count, setCount]; } starts with 'use' and uses camelCase correctly.

3. Final Answer:

   function useCounter() { const [count, setCount] = useState(0); return [count, setCount]; } -> Option A

4. Quick Check:

   Custom hook names start with 'use' [OK]

Hint: Custom hooks always start with 'use' prefix [OK]

Common Mistakes:

• Not starting function name with 'use'
• Using uppercase first letter in hook name
• Naming hooks like regular functions

3. Given this custom hook code, what will be logged when ExampleComponent renders?

function useToggle(initial) {
  const [on, setOn] = React.useState(initial);
  const toggle = () => setOn(!on);
  return [on, toggle];
}

function ExampleComponent() {
  const [isOn, toggle] = useToggle(false);
  React.useEffect(() => {
    console.log(isOn);
  }, [isOn]);
  return Toggle;
}

medium

A. undefined on first render, then false after button click

B. true on first render, then false after button click

C. No output because console.log is inside useEffect

D. false on first render, then true after button click

Solution

1. Step 1: Analyze initial state in useToggle

   The hook starts with initial = false, so isOn is false initially.

2. Step 2: Understand useEffect logging

   useEffect logs isOn after render; initially logs false, after toggle logs true.

3. Final Answer:

   false on first render, then true after button click -> Option D

4. Quick Check:

   Initial state false, toggles true [OK]

Hint: Initial state logs first, toggle changes it [OK]

Common Mistakes:

• Assuming initial state is true
• Thinking console.log won't run inside useEffect
• Confusing toggle function behavior

4. What is wrong with this custom hook code?

function useCounter() {
  const [count, setCount] = React.useState(0);
  function increment() {
    count = count + 1;
  }
  return { count, increment };
}

medium

A. Directly modifying count variable instead of using setCount

B. Missing React import for useState

C. increment function should return count

D. Custom hook name should not start with 'use'

Solution

1. Step 1: Check how state is updated

   State variables like count must be updated via setCount, not by direct assignment.

2. Step 2: Identify the error in increment

   increment tries to assign count = count + 1, which does not update React state.

3. Final Answer:

   Directly modifying count variable instead of using setCount -> Option A

4. Quick Check:

   State updates need setCount [OK]

Hint: Always use setState to update state, never assign directly [OK]

Common Mistakes:

• Trying to assign state variable directly
• Forgetting to call setCount
• Confusing variable assignment with state update

5. You want to create a custom hook useFetch that fetches data from a URL and returns loading state, data, and error. Which approach correctly reuses logic and handles updates safely?

function useFetch(url) {
  const [data, setData] = React.useState(null);
  const [loading, setLoading] = React.useState(true);
  const [error, setError] = React.useState(null);

  React.useEffect(() => {
    setLoading(true);
    fetch(url)
      .then(res => res.json())
      .then(json => {
        setData(json);
        setLoading(false);
      })
      .catch(err => {
        setError(err.message);
        setLoading(false);
      });
  }, [url]);

  return { data, loading, error };
}

hard

A. This hook will cause infinite loops because setState is inside useEffect

B. This hook should not use useEffect for fetching data

C. This hook correctly reuses fetch logic and updates state on URL change

D. This hook must return an array, not an object

Solution

1. Step 1: Check useEffect dependency

   useEffect runs when URL changes, so fetch updates correctly.

2. Step 2: Verify state updates inside fetch

   State updates happen after fetch resolves or errors, no infinite loops occur.

3. Step 3: Confirm return value format

   Returning an object with data, loading, error is a common pattern and valid.

4. Final Answer:

   This hook correctly reuses fetch logic and updates state on URL change -> Option C

5. Quick Check:

   useEffect fetches on URL change, updates state [OK]

Hint: useEffect with URL dependency safely fetches data [OK]

Common Mistakes:

• Thinking setState inside useEffect causes loops
• Not using useEffect for side effects
• Confusing return types of hooks