Cleanup function in React - Step-by-Step Execution

Concept Flow - Cleanup function

Component mounts

↓

useEffect runs

↓

Setup side effect

↓

Component updates or unmounts?

Yes→Cleanup function runs

↓

Side effect cleaned

↓

Component stays mounted

When a React component mounts, useEffect runs and sets up a side effect. On update or unmount, the cleanup function runs to remove or reset the effect.

Execution Sample

React

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

This code sets up a timer that logs 'tick' every second and cleans it up when the component unmounts.

Execution Table

Step	Trigger	Action	Effect Setup	Cleanup Function Called	Effect Cleanup
1	Component mounts	useEffect runs	setInterval started	No	No
2	1 second passes	Interval callback runs	No change	No	No
3	Component unmounts	Cleanup function runs	No change	Yes	clearInterval called
4	After unmount	No further interval callbacks	No change	No	No

💡 Component unmount triggers cleanup, stopping the interval and preventing further callbacks.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
id	undefined	interval ID assigned	interval ID unchanged	interval cleared	undefined or invalid

Key Moments - 3 Insights

Why does the cleanup function run when the component unmounts?

Because React calls the cleanup function returned by useEffect to remove side effects and avoid memory leaks, as shown in step 3 of the execution_table.

What happens if we forget to clear the interval in the cleanup?

The interval keeps running even after the component is gone, causing unexpected behavior or memory leaks, which is prevented by the cleanup in step 3.

Does the cleanup function run on every render?

No, it runs only before the next effect runs or when the component unmounts. Here, with an empty dependency array, it runs only on unmount (step 3).

Visual Quiz - 3 Questions

Look at the execution_table, what happens at step 3?

A The component mounts and sets up the interval

B The cleanup function runs and clears the interval

C The interval callback logs 'tick'

D Nothing happens

💡 Hint

Check the 'Cleanup Function Called' and 'Effect Cleanup' columns at step 3 in the execution_table

According to variable_tracker, what is the state of 'id' after step 3?

A Undefined

B Interval ID assigned and active

C Interval cleared and invalid

D Interval ID changed to a new value

💡 Hint

Look at the 'After Step 3' and 'Final' columns for 'id' in variable_tracker

If the dependency array was omitted, when would the cleanup function run?

A On every render before the next effect runs

B Only on component unmount

C Never

D Only once after the first render

💡 Hint

Recall React's useEffect behavior without dependencies, cleanup runs before each new effect

Concept Snapshot

useEffect(() => {
  // setup side effect
  return () => {
    // cleanup side effect
  };
}, [dependencies]);

Cleanup runs before next effect or on unmount.
Prevents memory leaks and stale effects.

Full Transcript

In React, the cleanup function inside useEffect runs to remove side effects like timers or subscriptions. When the component mounts, useEffect sets up the effect. When the component unmounts or before the effect runs again, React calls the cleanup function to clear or reset the effect. This prevents issues like memory leaks. For example, a timer set with setInterval is cleared in cleanup to stop it when the component is gone. If dependencies are empty, cleanup runs only on unmount. Without dependencies, cleanup runs before every new effect. This flow ensures React components manage resources safely and efficiently.