Edit: It just occurred to me that there's likely no need to reset the variable within the useEffect hook. In fact, stateTheCausesUseEffectToBeInvoked's actual value is likely inconsequential. It is, for all intents and purposes, simply a way of triggering useEffect.
Let's say I have a functional React component whose state I initialize using the useEffect hook. I make a call to a service. I retrieve some data. I commit that data to state. Cool. Now, let's say I, at a later time, interact with the same service, except that this time, rather than simply retrieving a list of results, I CREATE or DELETE a single result item, thus modifying the entire result set. I now wish to retrieve an updated copy of the list of data I retrieved earlier. At this point, I'd like to again trigger the useEffect hook I used to initialize my component's state, because I want to re-render the list, this time accounting for the newly-created result item.
const myComponent = () => {
const [items, setItems] = ([])
useEffect(() => {
const getSomeData = async () => {
try {
const response = await callToSomeService()
setItems(response.data)
setStateThatCausesUseEffectToBeInvoked(false)
} catch (error) {
// Handle error
console.log(error)
}
}
}, [stateThatCausesUseEffectToBeInvoked])
const createNewItem = async () => {
try {
const response = await callToSomeService()
setStateThatCausesUseEffectToBeInvoked(true)
} catch (error) {
// Handle error
console.log(error)
}
}
}
I hope the above makes sense.
The thing is that I want to reset stateThatCausesUseEffectToBeInvoked to false WITHOUT forcing a re-render. (Currently, I end up calling the service twice--once for win stateThatCausesUseEffectToBeInvoked is set to true then again when it is reset to false within the context of the useEffect hook. This variable exists solely for the purpose of triggering useEffect and sparing me the need to elsewhere make the selfsame service request that I make within useEffect.
Does anyone know how this might be accomplished?
There are a few things you could do to achieve a behavior similar to what you described:
Change stateThatCausesUseEffectToBeInvoked to a number
If you change stateThatCausesUseEffectToBeInvoked to a number, you don't need to reset it after use and can just keep incrementing it to trigger the effect.
useEffect(() => {
// ...
}, [stateThatCausesUseEffectToBeInvoked]);
setStateThatCausesUseEffectToBeInvoked(n => n+1); // Trigger useEffect
Add a condition to the useEffect
Instead of actually changing any logic outside, you could just adjust your useEffect-body to only run if stateThatCausesUseEffectToBeInvoked is true.
This will still trigger the useEffect but jump right out and not cause any unnecessary requests or rerenders.
useEffect(() => {
if (stateThatCausesUseEffectToBeInvoked === true) {
// ...
}
}, [stateThatCausesUseEffectToBeInvoked]);
Assuming that 1) by const [items, setItems] = ([]) you mean const [items, setItems] = useState([]), and 2) that you simply want to reflect the latest data after a call to the API:
When the state of the component is updated, it re-renders on it's own. No need for stateThatCausesUseEffectToBeInvoked:
const myComponent = () => {
const [ items, setItems ] = useState( [] )
const getSomeData = async () => {
try {
const response = await callToSomeService1()
// When response (data) is received, state is updated (setItems)
// When state is updated, the component re-renders on its own
setItems( response.data )
} catch ( error ) {
console.log( error )
}
}
useEffect( () => {
// Call the GET function once ititially, to populate the state (items)
getSomeData()
// use [] to run this only on component mount (initially)
}, [] )
const createNewItem = async () => {
try {
const response = await callToSomeService2()
// Call the POST function to create the item
// When response is received (e.g. is OK), call the GET function
// to ask for all items again.
getSomeData()
} catch ( error ) {
console.log( error )
}
} }
However, instead of getting all items after every action, you could change your array locally, so if the create (POST) response.data is the newly created item, you can add it to items (create a new array that includes it).
Related
I'm trying to save State twice, so I can reset it later on, but no matter what method I try, the 'setFullTrials' won't update with the saved data. The "console.log(savedData)" shows that all the data is there, so that's definitely not the problem. Not sure where I'm going wrong.
function AllTrials({Trialsprop}) {
let [savedData, setSavedData] = useState([]);
let [fullTrials, setFullTrials] = useState([]);
useEffect(() => {
//Call the Database (GET)
fetch("/trials")
.then(res => res.json())
.then(json => {
// upon success, update trials
console.log(json);
setFullTrials(json);
setSavedData(json);
})
.catch(error => {
// upon failure, show error message
});
}, []);
const resetState = () => {
setFullTrials(savedData);
//setFullTrials((state) => ({
...state,
savedData
}), console.log(fullTrials));
// setFullTrials(savedData.map(e => e));
console.log("savedData", savedData)
}
Setting the state in React acts like an async function.
Meaning that the when you set the state and put a console.log right after it, it will likely run before the state has actually finished updating.
Which is why we have useEffect, a built-in React hook that activates a callback when one of it's dependencies have changed.
Example:
useEffect(() => {
console.log(fullTrials)
// Whatever else we want to do after the state has been updated.
}, [fullTrials])
This console.log will run only after the state has finished changing and a render has occurred.
Note: "fullTrials" in the example is interchangeable with whatever other state piece you're dealing with.
Check the documentation for more info.
P.S: the correct syntax for useState is with const, not let.
i.e. - const [state, setState] = useState()
Trying to get a basic chat app going and having problems with excessive rerenders when a message is sent. Here is the applicable code for the client:
const [chatMessages, setChatMessages] = useState([]);
const sendChat = (e) => {
socket.emit("sendMessage", e.target.elements.message.value);
}
useEffect(() => {
socket.on("receiveMessage", (chatMessage) => {
setChatMessages([...chatMessages, chatMessage]);
console.log(chatMessages);
});
}, [chatMessages]);
return (
{chatMessages.map((message) => <p>{message}</p>)}
)
Then, on the server:
io.on("connection", (socket) => {
socket.on("sendMessage", (chatMessage) => {
console.log("message sent");
io.to(roomId).emit("receiveMessage", chatMessage);
});
}
When I do this, the message is successfully sent and received but it results in it happening lots of times (console):
[]
[]
[{...}]
[{...}]
(2) [{...}, {...}]
On the third message this is what gets logged. By the the sixth or seventh message the whole page comes to a grinding halt as it logs about 100 times.
I have tried the following:
Having an empty dependency array in the useEffect(). This does fix the rerenders, but introduces a new problem. The latest message is the only one that is saved and replaced the last one, so you can only see one message at a time.
Taking it out of useEffect() all together. This just worsens the problem and causes even more rerenders per message.
Any help would be appreciated. Thank you!
Issue
You are creating socket event handlers when the chatMessages state updates but not cleaning them up. If you edit your code or the component rerenders, etc.... then yet another socket event handler is added. The multiple handlers will start to stack up and enqueue multiple unexpected state updates.
Additionally, since React state updates are asynchronously processed you can't log the state immediately after enqueueing the update and expect to see the updated state. Use a separate useEffect hook for this.
Solution
Add an useEffect cleanup function to remove the event handler and re-enclose the updated chatMessages state array in the handler callback.
useEffect(() => {
const handler = (chatMessage) => {
setChatMessages([...chatMessages, chatMessage]);
}
socket.on("receiveMessage", handler);
return () => socket.off("receiveMessage", handler);
}, [chatMessages]);
Add an useEffect cleanup function, remove the dependencies so the effect runs once on component mount, and use a functional state update to correctly update from the previous state instead of the initial state in the callback enclosure.
useEffect(() => {
const handler = (chatMessage) => {
setChatMessages(chatMessages => [...chatMessages, chatMessage]);
}
socket.on("receiveMessage", handler);
return () => socket.off("receiveMessage", handler);
}, []);
Between the two the second option is the more optimal solution, but which you choose is your decision.
To log the chatMessages state updates:
useEffect(() => {
console.log(chatMessages);
}, [chatMessages]);
Since you have a dependency on chatMessages, every time the chatMessages changes, it creates a new listener. And that's why it becomes slower and slower as and when more messages come in.
You could do two things:
You maintain the chatMessages locally within the useEffect method. And you can spread that array and just call setChatMessages with the spread array. When you do this, you can remove the chatMessages dependency to useEffect and still have all messages. As a good practice, you should return a function that will remove the event listener when the component unmounts.
const [chatMessages, setChatMessages] = useState([]);
const sendChat = (e) => {
socket.emit("sendMessage", e.target.elements.message.value);
}
useEffect(() => {
let localMessages = [];
const callback = (chatMessage) => {
localMessages = [...localMessages, chatMessage];
setChatMessages(localMessages);
console.log(localMessages);
};
socket.on("receiveMessage", callback);
return () => {
socket.off("receiveMessage", callback);
}
}, []);
return (
{chatMessages.map((message) => <p>{message}</p>)}
)
You can probably use useRef for storing the values. However that will not trigger the re-render of the UI when the value changes and probably that's not what you want. So this may not be a good choice.
I a setState function being passed down from a parent component, I want to setTheState of the parent setter if the enterKey is pressed. Though, when I set the state nothing happens and I'm still left with an empty array
Here's the code snippet
const { check, setCheck } = props // receive from props
const callApi = async (onEnter) => {
const res = await callFunction('', data)
if (res) {
setResults(res)
if (onEnter) {
setCheck(res)
console.log('check', check) // returns []
}
}
Returns [] when I log in parent as well
Instead of console logging it immediately, wait for it to change, because it's an async operation. You can wrap your log in a useEffect:
useEffect(() => {
console.log(check);
}, [check]);
I have a state variable called list that updates when setList is called. SetList lives under the function AddToList, which adds a value to the existing values in the list. As of this moment, the function handleList executes prior to the state variable setList even though I have setList added prior to the function handleList. What I am trying to achieve is for the setList to update its list prior to running the handleList. Could you provide insights on how to fix this?
If you want to test the code, https://codesandbox.io/s/asynchronous-test-mp2fq?file=/Form.js
export default function Form() {
const [list, setList] = useState([]);
const addToList = (name) => {
let newDataList = list.concat(name);
setList(newDataList);
console.log("List: ", list);
handleList();
};
const handleList = async () => {
console.log("Handle List Triggered");
await axios
// .put("", list)
.get("https://api.publicapis.org/entries")
.then((response) => {
console.log("Response: ", response);
})
.catch((error) => {});
};
return (
<AutoComplete
name="list"
label="Add to List"
onChange={(events, values) => {
addToList(values.title);
}}
/>
);
}
As you can tell, the get response is made prior to updating the list.
It's not clear what you want to do with the updated list, but you know what the new list will be, so you can just pass that around if you need it immediately.
const addToList = (name) => {
let newDataList = list.concat(name);
setList(newDataList);
console.log("List: ", list);
handleList(newDataList);
};
const handleList = async (list) => {
console.log("Handle List Triggered");
await axios
// .put("", list)
.get("https://api.publicapis.org/entries")
.then((response) => {
console.log("Response: ", response);
})
.catch((error) => {});
};
React's useEffect hook has an array of dependencies that it watches for changes. While a lot of the time it's used with no dependencies (i.e. no second parameter or empty array) to replicate the functionality of componentDidMount and componentDidUpdate, you may be able to use that to trigger handleList by specifying list as a dependency like this:
useEffect(() => {
handleList(list);
}, [list]);
I think there may be a redundant request when page loads though because list will be populated which you'll most likely want to account for to prevent unnecessary requests.
first of all you have to understand setState is not synchronized that means when you call setList(newDataList) that not gonna triggered refer why setState is async
therefore you can use #spender solution
or useStateCallback hook but it's important understand setState is not sync
const [state, setState] = useStateCallback([]);
const addToList = (name) => {
........... your code
setList(newDataList, () => {
// call handleList function here
handleList();
});
}
i have set one product details to setProduct use state in function fetchProduct().
const [ product, setProduct ] = useState({})
const fetchProduct = () => {
const apiURL = //api url;
fetch(apiURL)
.then((response) => response.json())
.then(res => {
setProduct(res.data[0])
})
.catch(error => {
console.error(error);
});
}
after i have call this function inside useEffect hook. and i have checked product is set or no in call with colsole log
useEffect(() => {
fetchProduct()
console.log(product)
return () => {
}
},[])
following result i have get in console
{}
then i have refreshed again result is showing in console. why in first time result is empty object. thanks for help
fetch product is asynchronous as is setState your console log executes before the data is retrieved and set
There are several behaviors for useEffect and it's dependency array:
In your example you use an empty array which acts as- componentDidMount and runs once.
that's why when you refresh your component it doesn't hold the default value anymore which is {}, but the value that was changed by your function.
In this case i'd add product state to the dependecy array which will cause the useEffect to execute the code inside the useEffect once on mount and whenever the
product state changes
for more https://reactjs.org/docs/hooks-effect.html
In your useEffect when javascript start executing
FetchProduct it continue to compile the next line it wont stop till the function execution finishes and then continue.
If you want to access product and do whatever you want with it you can use another use Effect right after yours.
useEffect(() => {
// Here you can access the product
console.log(product);
}, [product])