Creating custom hooks in React - Visual Walkthrough

Concept Flow - Creating custom hooks

Start Component

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Call Custom Hook

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Inside Hook: useState/useEffect

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Return State/Functions

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Component Uses Hook Data

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Render UI with Hook Data

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User Interaction -> State Update

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Re-render Component with New State

The component calls the custom hook, which manages state and effects, returns data and functions, and the component uses these to render and update UI.

Execution Sample

React

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

function Counter() {
  const { count, increment } = useCounter();
  return <button onClick={increment}>{count}</button>;
}

A custom hook useCounter manages a count state and increment function, used by Counter component to display and update count on button click.

Execution Table

Step	Action	State Before	State After	Component Render Output
1	Component Counter mounts, calls useCounter	count: undefined	count: 0	<button>0</button>
2	User clicks button, increment called	count: 0	count: 1	<button>1</button>
3	Component re-renders with updated count	count: 1	count: 1	<button>1</button>
4	User clicks button again, increment called	count: 1	count: 2	<button>2</button>
5	Component re-renders with updated count	count: 2	count: 2	<button>2</button>
6	No more clicks, component stable	count: 2	count: 2	<button>2</button>

💡 Execution stops when user stops interacting; state remains stable.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
count	undefined	0	1	2	2

Key Moments - 3 Insights

Why does the component re-render after calling increment?

Because increment updates the state 'count' inside the custom hook, React triggers a re-render to update the UI, as shown in steps 2 and 3 of the execution_table.

Is the custom hook called multiple times or just once?

The custom hook is called on every render of the component, but React preserves the state between renders. This is why 'count' updates correctly as seen in the variable_tracker.

Can the custom hook return anything other than state and functions?

Yes, it can return any value needed by the component, but typically it returns state and functions to manage that state, as shown in the example where it returns { count, increment }.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'count' after Step 2?

A 1

B 0

C 2

D undefined

💡 Hint

Check the 'State After' column for Step 2 in the execution_table.

At which step does the component first render with count equal to 0?

A Step 1

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Component Render Output' column in the execution_table.

If the increment function did not update state, what would happen to the component render output?

A It would update normally

B It would never re-render with new count

C It would crash

D It would show an error message

💡 Hint

Refer to how state changes trigger re-renders in the execution_table and key_moments.

Concept Snapshot

Creating custom hooks:
- Define a function starting with 'use'
- Use React hooks inside (useState, useEffect)
- Return state and functions
- Call hook inside functional components
- State updates trigger re-renders
- Reuse logic cleanly across components

Full Transcript

This visual execution shows how a React component uses a custom hook to manage state. The custom hook useCounter initializes a count state to zero and provides an increment function. When the component mounts, it calls useCounter, which returns the current count and increment. The component renders a button showing the count. When the user clicks the button, increment updates the count state inside the hook. React detects this state change and re-renders the component with the new count value. The execution table traces each step: mounting, clicking, state updates, and re-renders. The variable tracker shows how the count changes from undefined to 0, then increments with each click. Key moments clarify why re-renders happen and how hooks preserve state across renders. The quiz tests understanding of state values at steps and the effect of state updates on rendering. The snapshot summarizes how to create and use custom hooks to share logic and state in React components.