Effect execution timing in React

Introduction

Effects let your React components do extra work like fetching data or updating the page. Knowing when effects run helps you control your app's behavior smoothly.

When you want to fetch data right after a component appears on the screen.

When you need to update the page title or document after some data changes.

When you want to set up or clean up timers or subscriptions as components mount or unmount.

When you want to run code only once when the component loads.

When you want to run code every time some specific data changes.

Syntax

React

useEffect(() => {
  // code to run
  return () => {
    // cleanup code
  };
}, [dependencies]);

The function inside useEffect runs after the component renders.

The optional dependencies array controls when the effect runs again.

Examples

This effect runs after every render because there is no dependencies array.

React

useEffect(() => {
  console.log('Runs after every render');
});

This effect runs only once because the dependencies array is empty.

React

useEffect(() => {
  console.log('Runs only once after first render');
}, []);

This effect runs only when the count value changes.

React

useEffect(() => {
  console.log('Runs when count changes:', count);
}, [count]);

This effect sets a timer once and cleans it up if the component unmounts.

React

useEffect(() => {
  const timer = setTimeout(() => {
    console.log('Timer done');
  }, 1000);
  return () => clearTimeout(timer);
}, []);

Sample Program

This component shows a count and a button to increase it. The effect runs after the count changes, updates the page title, and logs messages. It also cleans up before running again.

React

import React, { useState, useEffect } from 'react';

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

  useEffect(() => {
    console.log('Effect runs: count is', count);
    document.title = `Count: ${count}`;

    return () => {
      console.log('Cleanup for count', count);
    };
  }, [count]);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increase</button>
    </div>
  );
}

export default TimerComponent;

Important Notes

Effects run after the browser paints the screen, so they don't block showing the UI.

Always include dependencies to avoid running effects too often or missing updates.

Cleanup functions help avoid bugs like memory leaks or duplicated timers.

Summary

Effects run after rendering to do extra work.

Dependencies array controls when effects run again.

Cleanup functions run before the effect re-runs or component unmounts.

Practice

1. In React, when does the useEffect hook run its effect function by default?

easy

A. Only when the component unmounts

B. Before the component renders

C. Only once when the component mounts

D. After every render of the component

Solution

1. Step 1: Understand default useEffect behavior

   By default, useEffect runs after every render, including the first render and all updates.

2. Step 2: Compare with other lifecycle timings

   It does not run before render or only once unless dependencies are specified.

3. Final Answer:

   After every render of the component -> Option D

4. Quick Check:

   Default effect runs after render [OK]

Hint: Default useEffect runs after every render unless dependencies given [OK]

Common Mistakes:

• Thinking useEffect runs before render
• Assuming it runs only once without dependencies
• Confusing cleanup timing with effect timing

2. Which of the following is the correct syntax to run a useEffect only once when the component mounts?

easy

A. useEffect(() => { /* effect */ }, [true])

B. useEffect(() => { /* effect */ })

C. useEffect(() => { /* effect */ }, [])

D. useEffect(() => { /* effect */ }, [null])

Solution

1. Step 1: Identify dependency array usage

   An empty array [] means the effect runs only once after the first render (mount).

2. Step 2: Check other options

   Omitting dependencies runs every render; non-empty arrays cause effect to run on changes.

3. Final Answer:

   useEffect(() => { /* effect */ }, []) -> Option C

4. Quick Check:

   Empty dependency array = run once on mount [OK]

Hint: Empty array runs effect once on mount [OK]

Common Mistakes:

• Omitting dependency array runs effect every render
• Using non-empty arrays thinking it runs once
• Confusing null or true as empty array

3. Consider this React component:

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

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

medium

A. Effect: 0, Cleanup: 0, Effect: 1, Cleanup: 1, Effect: 2

B. Effect: 0, Effect: 1, Effect: 2

C. Effect: 0, Cleanup: 0, Cleanup: 1, Effect: 2

D. Cleanup: 0, Cleanup: 1, Effect: 2

Solution

1. Step 1: Understand effect and cleanup with dependencies

   Effect runs after mount and after each count change. Cleanup runs before effect re-runs or unmount.

2. Step 2: Trace logs on two clicks

   Initial mount: Effect: 0
   First click: Cleanup: 0, Effect: 1
   Second click: Cleanup: 1, Effect: 2

3. Final Answer:

   Effect: 0, Cleanup: 0, Effect: 1, Cleanup: 1, Effect: 2 -> Option A

4. Quick Check:

   Effect runs after render; cleanup before next effect [OK]

Hint: Cleanup runs before next effect when dependencies change [OK]

Common Mistakes:

• Ignoring cleanup logs
• Assuming cleanup runs after effect
• Missing initial effect run on mount

4. What is wrong with this React effect code?

React.useEffect(() => {
  const id = setInterval(() => console.log('Tick'), 1000);
  return clearInterval(id);
}, []);

medium

A. The effect will run multiple times because dependencies are missing

B. The cleanup function should be a function, not the result of clearInterval

C. setInterval should be replaced with setTimeout

D. The effect will never run because of missing dependencies

Solution

1. Step 1: Check cleanup function syntax

   The cleanup must be a function returning a function, but here it calls clearInterval immediately.

2. Step 2: Correct cleanup usage

   It should return a function: return () => clearInterval(id); to run cleanup on unmount.

3. Final Answer:

   The cleanup function should be a function, not the result of clearInterval -> Option B

4. Quick Check:

   Cleanup must be a function returning a function [OK]

Hint: Cleanup must be a function, not a direct call [OK]

Common Mistakes:

• Returning cleanup call instead of function
• Confusing dependencies with cleanup
• Thinking setInterval needs setTimeout

5. You want a React effect to run only when either propA or stateB changes, but not on every render. Which dependency array is correct?

React.useEffect(() => {
  // effect code
}, ???);

hard

A. [propA, stateB]

B. []

C. [propA || stateB]

D. [propA && stateB]

Solution

1. Step 1: Understand dependency array purpose

   The effect runs when any value in the array changes compared to last render.

2. Step 2: Analyze options

   [propA, stateB] lists both dependencies separately, so effect runs if either changes.
   Options C and D use logical operators, which evaluate to single values and won't track changes properly.
   [] runs effect only once on mount.

3. Final Answer:

   [propA, stateB] -> Option A

4. Quick Check:

   List all dependencies separately in array [OK]

Hint: List each dependency separately in array to track changes [OK]

Common Mistakes:

• Using logical operators inside dependency array
• Leaving dependency array empty when dependencies exist
• Assuming single combined value tracks all changes