Common lifecycle use cases in React - Step-by-Step Execution

Concept Flow - Common lifecycle use cases

Component Mounts

↓

useEffect with [

↓

Component Renders

↓

User Interaction or Prop Change

↓

State Update triggers Re-render

↓

useEffect with dependencies runs

↓

Component Unmounts

↓

Cleanup function runs

This flow shows how React functional components use useEffect for mounting, updating, and cleanup during their lifecycle.

Execution Sample

React

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

function Timer() {
  const [count, setCount] = useState(0);
  useEffect(() => {
    const id = setInterval(() => setCount(c => c + 1), 1000);
    return () => clearInterval(id);
  }, []);
  return <div>Seconds: {count}</div>;
}

A timer component that increments seconds every second using useEffect to start and clean up the interval.

Execution Table

Step	Trigger	State Before	Action	State After	What Re-renders	DOM Change
1	Initial Mount	count=0	Run useEffect (start interval)	count=0	Timer component	Display 'Seconds: 0'
2	1 second passes	count=0	Interval callback increments count	count=1	Timer component	Display 'Seconds: 1'
3	2 seconds pass	count=1	Interval callback increments count	count=2	Timer component	Display 'Seconds: 2'
4	Component Unmounts	count=2	Cleanup function clears interval	count=2	No re-render	Timer removed from DOM
5	No further intervals	count=2	No action	count=2	No re-render	No DOM change

💡 Component unmounts and cleanup stops interval, no further state updates.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
count	0	1	2	2	2

Key Moments - 3 Insights

Why does the interval keep running even though the component re-renders multiple times?

Because the useEffect has an empty dependency array [], it runs only once on mount, so the interval is set up once and keeps running independently of re-renders (see execution_table steps 1-3).

What happens if we forget to return the cleanup function in useEffect?

The interval would continue running even after the component unmounts, causing memory leaks and unexpected behavior (see execution_table step 4 for cleanup importance).

Why does the component re-render when count changes?

Because setCount updates state, React triggers a re-render to update the displayed count (see execution_table steps 2 and 3).

Visual Quiz - 3 Questions

Look at the execution table, what is the value of count at step 3?

A 0

B 1

C 2

D 3

💡 Hint

Check the 'State After' column at step 3 in the execution_table.

At which step does the cleanup function run to stop the interval?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for 'Cleanup function clears interval' in the 'Action' column.

If the dependency array in useEffect was omitted, how would the execution table change?

A The interval would start only once on mount.

B The interval would restart on every render, creating multiple intervals.

C The cleanup would never run.

D The count would never update.

💡 Hint

Think about how useEffect runs without dependencies and how that affects intervals.

Concept Snapshot

React functional components use useEffect to handle lifecycle events.
- useEffect with [] runs once on mount (setup).
- Return a cleanup function to run on unmount.
- State updates trigger re-renders.
- useEffect with dependencies runs on those changes.
This manages side effects like timers safely.

Full Transcript

This visual execution shows how React functional components use the useEffect hook to manage common lifecycle use cases. When the component mounts, useEffect runs once to start a timer interval. The state variable 'count' increments every second, triggering re-renders that update the displayed seconds. When the component unmounts, the cleanup function returned by useEffect clears the interval to prevent memory leaks. The execution table traces each step: initial mount, timer ticks, and cleanup on unmount. The variable tracker shows how 'count' changes over time. Key moments clarify why useEffect runs only once with an empty dependency array, why cleanup is necessary, and how state updates cause re-renders. The quiz tests understanding of state values at steps, when cleanup runs, and effects of missing dependencies. This helps beginners see React lifecycle in action with hooks.