I have been trying to use componentDidMount on a project but it only appears to work when the file uses classes.
With hooks you can use useEffect to get same result.
This one would work like componentDidMount
useEffect(() = > {
}, [])
If you put a callback in empty array then it would work when you use that callback
According to the React docs:
You can’t use Hooks inside a class component, but you can definitely
mix classes and function components with Hooks in a single tree.
Instead of componentDidMount you can use useEffect instead, with an empty array as its dependency which will serve the same purpose of running code when the component mounts:
useEffect(() = > {
// ... code
}, [])
The above answer:
With hooks you can use useEffect to get same result.
This one would work like componentDidMount
useEffect(() = > {
}, [])
is right except:
If you put a callback in empty array then it would work when you use that callback
that empty array is where you would list dependencies that upon change would re-run the function. The empty array turns it into run only once.
Related
I have a useEffect where currently get from the redux the data and also clean up it during the unmounting:
useEffect(
dispatch(getData...)
() => {
dispatch(cleanData...)
},
[url]
);
When I set the url as dependency for using as componentDidupdate and updating the component if url is changed it throws some warnings in the console about not being able to work with unmounted component while functionality seems to work. What is the ideal way to have these 3 lifecycle methods in the same place for the useEffect?
useEffect expects two arguments. Callback Function that will get called at the initial render and when the dependencies gets changed. Second argument is dependency array.
So you can call dispatch function on initial render and on change of url . You have to return a function that is a cleanup function.
useEffect(() => {
dispatch(getData)
return () => {
dispatch(cleanData)
}
},[url]);
I want to trigger An API to do some actions when the value in the react state be empty, Is there a way in react state hook to achieve that?
If you are using a functional component you can use the "useEffect" hook with a proper dependency.
Class base components you might choose (if I understand your situation properly) something like the "componentWillUnmount()" method.
You could have a useEffect hook with the state as a dependency, in-which you can check if the state is empty and act accordingly.
Example:
const [state, setState] = useState([]);
useEffect(() => {
if (state.length !== 0) {
return;
}
// Can make your API call here (preferably call a function that does it)
// and then set the state.
setState(...);
}, [state]);
useEffect is a hook that accepts a callback as the first argument, and a dependency array as the second.
The callback will execute upon re-render whenever any of the dependencies in the array change.
Note: this is relevant only for functional components, for class-based component we have the componentDidUpdate lifecycle hook.
Am using useEffect in a react functional component to fetch data from an external API but it keeps calling the API endpoint on render on the page .
Am looking for a way to stop the useeffect from running on render on the component
Use the dependency array (second argument to the useEffect), so you can specify when you need to run the useEffect.
The problem here is that you have not used the dependency array, so that it executes every time. By adding a dependency array, you specify the changes where you want useEffect to run.
useEffect(()=>{
},[<dependency array: which contains the properties>]);
If you leave the dependency array empty, it will run only once. Therefore if you want the API call to run only once, add an empty array as the second argument to your useEffect. This is your solution.
Like this:
useEffect(()=>{
//Your API Call
},[]);
useEffect is always meant to run after all the changes or render effects are update in the DOM. It will not run while or before the DOM is updated. You may not have given the second argument to useEffect, which if u do not provide will cause the useEffect to execute on each and every change. Assuming you only want to call the API just once when on after the first render you should provide an empty array.
Runs on all updates, see no second argument to useEffect:
useEffect(() => { /* call API */ });
Runs when the prop or state changes, see the second argument:
useEffect(() => { /* call API */ }, [prop, state]);
Runs only once, see the empty second argument:
useEffect(() => { /* call API */ }, []);
I recommend you to read the full documentation about the React useEffect hook.
Here is a easy example of using useEffect
function functionalComponent() {
const [data, setData] = React.useState(null);
React.useEffect(() => {
const url = 'https://randomuser.me/api/?results=10';
fetch(url)
.then(data => {
setData(data);
})
.catch(error => console.error(error))
}, []); // it's necessary to use [] to avoid the re-rendering
return <React.Fragment>
{data !== null && (
<React.Fragment>
{data.results.map(data => (
<div>
{data.gender}
</div>
))}
</React.Fragment>
)}
</React.Fragment>;
}
Maybe in your useEffect implementation you are avoiding the [] dependencies, this is a bit hard to understand if you come from class states. This on hooks review when a state element inside the hook change, for example if you are using an element that always change like a prop that you pass throught another component you might be setting inside the dependencies or another state, if you do not need any dependency just use it empty like the example above. As you can see in the documentation sometimes the dependencies are not used, this might generate an infinite loop.
I want a line of code in a react component to be executed only when the component is mounted, and not when it re-renders. In a stateful component, I would do this in componentDidMount/componentWillMount method.
How can I do this using hooks?
I believe you are looking for reacts useEffect hook. It is the hook replacement for componentDidMount, componentDidUpdate, and componentWillUnmount. You can use it like this:
useEffect(() => {
console.log("only executed once")
}, [])
Note the empty array as second argument, which defines the dependencies for this effect. In this case you want to have no dependency, so that the effect wont be executed again.
There is a good article on how to replace the previous lifecycle functions with hooks available here.
// Similar to componentDidMount and componentDidUpdate:
useEffect(() => {}, [])
React Docs. Hooks-Using the Effect Hook. - > https://reactjs.org/docs/hooks-effect.html
I was going through React Hooks docs and it mentioned
If you’re familiar with React class lifecycle methods, you can think
of useEffect Hook as componentDidMount, componentDidUpdate, and
componentWillUnmount combined.
Suppose I have a class component right now where in componentDidMount I am doing something like this
componentDidMount() {
MapboxGL.setTelemetryEnabled(false);
}
As far as I can recall, Component did mount is only called once in lifecycle?
If I were to use react hooks then it would be something like this
useEffect(() => {
MapboxGL.setTelemetryEnabled(false);
});
This would call my function everytime state changes in react functional hooks component? Wouldn't it be redundant to call MapboxGL.setTelemetryEnabled(false); to call this everytime? when you only want to do it once component have mounted?
React docs have showed how useEffect can replace multiple lifecycle methods but I am still unable to comprehend how react hooks can replace componentDidMount?
Also, Just a side note question, Can you make a full fledge app using hooks (something like foursquare or instagram?)
You need to add a dependency array for it to know when to recall this hook. An empty dep array will only call it once aka "on mount". And if you don't provide a dep array then it will just be called on every re-render.
useEffect(() => {
MapboxGL.setTelemetryEnabled(false);
}, []);
You can create a flag and if the flag is false/true then only perform that action. something as simple as this
useEffect(() => {
if (something) {
MapboxGL.setTelemetryEnabled(false);
setSomething(false)
}
});
or if you only need hook once, you can do what Matt have suggested
useEffect(() => {
MapboxGL.setTelemetryEnabled(false);
}, []);