useEffect is for side effects.

Although the new hooks are considered “functional React,” they still follow Object-Oriented Programming methodologies. For example, useState maps to the pre-existing this.state of the older class components. State was a pretty easy concept to understand. Much like a class instance has its own variables (commonly accessed in JS with this), a React component has its own state.

Following Object-Oriented programming paradigms, you will have functions that affect outside classes. This is similar, conceptually, to useEffect.

When would we want to trigger some sort of side effect? Well, there are three general cases:

  • when the component is first created
  • whenever the component is re-rendered
  • when the component is destroyed
  • when the state changes (or part of the state)

Each of these changes how useEffect is called. Remember that the hook accepts two arguments:

useEffect(callback[, dependencies]);

If you pass in an empty array, useEffect() will be called only on the initial component creation (like componentDidMount for the class component). For example, if we wanted to make a request on creation, and fill the state, it would look something like this:

const [contents, setContents] = useState([])

useEffect(() => {
.then(res => res.json())
.then(json => setContents(json))
}, [])

This will populate the component, but only when it is first created.

If we want the useEffectcontents to run on every render, you should only pass in a callback, with no dependencies. For example, if a user is composing a message, you could send a copy of the draft to the server, so it does not get lost.

const [draft, updateDraft] = useState('')useEffect(() => {
fetch(url, {
method: 'POST',
body: draft,

Note that there is no second argument in this case.

Sometimes, when you set something up on component creation, you want to remove it after destruction. For example, you may want to update a Redux variable or close a websocket upon deletion of the component. This has little to do with the second argument of the function. Instead, you return a cleanup function in the callback. Note that this will run on every re-render, prior to the re-render.

useEffect(() => {
/* do something */
const cleanup = () => {
/* delete something */
return cleanup

For example, if you want to create a continuous function that logs to console every few seconds, you should delete it when the component is deleted:

useEffect(() => {
const id = setInterval(() => {
console.log("This is logged to console.");
}, 5000);
const cleanup = () => clearInterval(id)
return cleanup
}, []);

This is probably the most common use, besides when the component mounts. If you want to create a side effect that is based on the state, you pass in one or more pieces of state (or props) that you care about.

```javascript const ToyReactComponent = (props) => { const [internalState, changeInternalState] = useState(“”)

useEffect(() => { console.log(“Something was updated!”) console.log(internalState, props.somePropValue) }, [internalState, props.somePropValue]) }

If any variable is used in useEffect, it should almost ALWAYS be in the dependency list.

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