So basically this is i what i want to do. I have a button which calls the handleAdd event, when that event triggers i want to call the useEffect function. The useEffect(); calls a function which returns a different api key depending on the value in input. But i have realized useEffect doesen't work that way and can only be called in the "top level" of the code. Is there some way i can work around this issue? See example below:
input = "example";
const handleAdd = () => {
//Call useEffect
};
useEffect(() => {
getCoinMarketDataApi.request("example");
}, []);
Thanks in advance:) any input is appreciated, i'm having quite the hard time trying to figure out how to work with useEffect and async events so the answer might be obvious.
It sounds like the function that you have in useEffect should really be part of the function that gets passed to the <button>s onClick handler.
There are use cases when you do need to retrigger a useEffect in response to a button click (e.g. a project I’m working on needs to fetch data from a web service when the component is mounted and again if user input changes a value from the default) so:
useEffect functions get called when:
The component is initially mounted
When a dependency changes
So you can do what you are asking for by:
Creating a state with useState
Passing the state variable as a dependency to useEffect
Setting that state variable in your click event handler
Related
This question already has answers here:
The useState set method is not reflecting a change immediately
(15 answers)
Closed 2 months ago.
I am using react-query, and there is a useCallback function dependent on some data from useQuery, but when I call useQuery's refetch funciton, the data in useCallback not update. I have done a test in useEffect, and the data in useEffect has updated.
useCallback
const updateCurrentActiveTemplate = useCallback(
(type: string) => {
console.log(templatesListQuery.data);
},
[templatesListQuery.data]
);
useEffect
useEffect(() => {
console.log(templatesListQuery.data, 'effect');
}, [templatesListQuery.data]);
the refetch part
const handleTemplateDataSubmit = () => {
await templatesListQuery.refetch();
updateCurrentActiveTemplate(currentBusinessType);
}
When I click the submit button I will call the handleTemplateDataSubmit method.
When I call useQuery's refetch function, the two logs are all executed, but the data in useCallback and useEffect are different, data in useCallback is the stale data and data in useEffect is updated, why is that.
When you create handleTemplateDataSubmit it binds the updateCurrentActiveTemplate creating a closure. When you call it, it still has the reference to the same function. Next handleTemplateDataSubmit, created on the next rerender of the component will bind the updated version of updateCurrentActiveTemplate which in turn will bind the new data.
useEffect is called after the component rerenders and so the function that is passed to it binds new data.
You can call updateCurrentActiveTemplate inside useEffect it should work. You could also, for testing purposes add another button which call the updateCurrentActiveTemplate and then press first submit and then the second button. Second button will have the new updateCurrentActiveTemplate function.
Edit
The refetch function should return the updated results. It's probably the cleanest solution.
I was going through useEffect from reactjs docs and I've found this statement here
Experienced JavaScript developers might notice that the function
passed to useEffect is going to be different on every render.
We are passing a function to useEffect and this function is said to be different for each render. useEffect has access to state and props since it's inside the function component and when either of these changes, we can see that change in the function of useEffect(because of closure) right? This is not clear, because in the next line the doc states
This is intentional. In fact, this is what lets us read the count
value from inside the effect without worrying about it getting stale.
To counter this, assume we have a function
function foo(n) {
bar = () => {
setTimeout(() => console.log({n}), 50);
return n;
}
setTimeout(() => {n = 10}, 0);
setTimeout(() => {n = 20}, 100);
setTimeout(() => {n = 30}, 150);
return bar;
}
baz = foo(1);
baz(); //prints 10
setTimeout(baz, 300); //prints 30
It seems that when the closure value(n) is changed, we can see that change in the setTimeout's callback (and this callback isn't changed over time). So, how can the closured value(state/props) in useEffect's function become stale as mentioned in docs?
Am I missing something here? I think it's more of a JS question compared to React, so I took a JS example.
I found the answer a few days back, and as #apokryfos(Thank you again!) mentioned in the comments above, the program execution process is now making more sense. I want to summarize my learnings here.
Firstly, the code I considered, was not like with like comparison (in #apokryfos words) with the React doc statements, and this is true. In case of static HTML + vanilla JS where the HTML button has an event-listener JS function, this function is declared only once and when the event occurs the same JS function is executed everytime.
The code I have given in the question is similar to this, and so when executed in console or in event listener will only be declared once.
In case of React(or any state based UI libraries/frameworks), the HTML is not static and it needs to change on state-change. On the execution side (considering React), component will be created when we call it (in JSX), and the component's function/class will be executed completely from top to bottom. This means
from all the event-handlers that doesn't deal with state, constants to useState's destructed elements and useEffect's callback functions, everything are re-initialized.
If the parent's state changes initiate a render on its children(in normal scenarios), then the children will need to re-render themselves completely with the new props and/or new state to show the updated UI
Considering the example in React docs (link), useEffect with no dependencies will get executed after every render, and here it's updating the DOM by showing the current state value. Unless the callback function has the latest value of that state, it'll only print the stale value. So re-initialising the functions here is the main reason behind not having stale values in the callback functions
function Example() {
const [count, setCount] = useState(0);
useEffect(() => {
document.title = 'You clicked ${count} times';
});
}
This is a boon and a curse sometimes. Assume if we are sending useState's setState function to the child, and in the child, we have a useEffect(that makes a network call to fetch data) that takes this function as its dependency. Now, for every state change in parent, even if there is a use-case or not, the useEffect will get triggered as this function in dependency is changed (as every update re-initializes the functions). To avoid this, we can utilize useCallback on the functions which we want to memorize and change only when certain params are changed, but it is not advisable to use this on useEffect's callback function since we might end-up in stale values.
Further Reading:
GeeksForGeeks useCallback
SourceCode interpretation of useEffect
Another SourceCode interpretation of useEffect
I have a screen with some choices on. If you select the choice it sets state of the data. I then have a confirm button. if the user hits confirm I make an async call to get some extra data. I want to wait for this to happen before opening the modal as I need to present that extra data in my modal.
before hooks I would use setState and do something like:
this.setState({data: myData}, () => this.openModal()) as this would reliably set the state then open the modal. all the answers online seem to suggest using useEffect but it seems dodgy to do this:
useEffect(() => {
if (data) {
setModalOpen(true)
}
}, [data, setData])
I don't want my modal potentially randomly opening at different points. plus it seems better to have the code living in the same place I set state. it makes sense to be there. not some random useEffect
any suggestions how this can be achieved?
(one other solution I can think of is making the API call on every choice select, rather than before confirm) however, this could lead to a lot of unnecessary API calls so I'd rather not go down that route.
Using useEffect() is correct, I also encountered this issue when trying to do a callback on setState with hooks.
Like you said: this.setState({data: myData}, () => this.openModal()) was possible before, but now when trying this with hooks the console displays the error:
Warning: State updates from the useState() and useReducer() Hooks don't support the second callback argument. To execute a side effect after rendering, declare it in the component body with useEffect().
So useEffect() seems the way to go.
You should use useEffect() as a callback after the state is correctly setted if you would like to do something with the state like validation.
useEffect(() => {
// fetch on load
axios.get("https://randomuser.me/api/").then((response) => {
setPerson(response.data.results[0]);
});
}, []);
useEffect(() => {
// do some validation perhaps
if (person !== null) {
if (person.name.first && person.name.last) {
setModal(true);
} else {
setModal(false);
}
}
}, [person]); // add person in dependency list
As suggested in the comments, you could also do setModal() when the async data has arrived (using .then() or await).
Some example code using random user generator API and axios for fetching.
useEffect(() => {
// fetch on load
axios.get("https://randomuser.me/api/").then((response) => {
setPerson(response.data.results[0]);
setModal(true); // set modal visibility
});
}, []);
setState updates state asynchronously. It's my understanding that, when using a class component, you can do something like this to ensure certain code is executed after one setState updates state:
setState({color: red}, callbackThatExecutesAfterStateIsChanged);
I'm using a functional component & hooks. I'm aware, here, useEffect()'s callback will execute everytime after color state changes and on initial execution.
useEffect(callback, [color]);
How can I replicate similar behaviour as the class component example - that is, to execute a chunk of code once after one setState() successfully changes state and not on initial execution?
If you ask me, there is no safe way to do this with hooks.
The problem is that you both have to read and set an initialized state in order to ignore the first update:
const takeFirstUpdate = (callback, deps) => {
const [initialized, setInitialized] = useState(false);
const [wasTriggered, setWasTriggered] = useState(false);
useEffect(() => {
if (!initialized) {
setInitialized(true);
return;
}
if (wasTriggered) {
return;
}
callback();
setWasTriggered(true);
}, [initialized, wasTriggered]);
};
While the hook looks like it works, it will trigger itself again by calling setInitialized(true) in the beginning, thus also triggering the callback.
You could remove the initialized value from the deps array and the hook would work for now - however this would cause an exhaustive-deps linting error. The hook might break in the future as it is not an "official" usage of the hooks api, e.g. with updates on the concurrent rendering feature that the React team is working on.
The solution below feels hacky. If there's no better alternative, I'm tempted to refactor my component into a class component to make use of the easy way class components allow you to execute code once state has been updated.
Anyway, my current solution is:
The useRef(arg) hook returns an object who's .current property is set to the value of arg. This object persists throughout the React lifecycle. (Docs). With this, we can record how many times the useEffect's callback has executed and use this info to stop code inside the callback from executing on initial execution and for a second time. For example:
initialExecution = useRef(true);
[color, setColor] = useState("red");
useEffect(() => {
setColor("blue");
});
useEffect(() => {
if (initialExecution.current) {
initialExecution.current = false;
return;
}
//code that executes when color is updated.
}, [color]);
index.js:1 Warning: State updates from the useState() and useReducer()
Hooks don't support the second callback argument. To execute a side
effect after rendering, declare it in the component body with
useEffect().
This is the error I get when trying to do this
setUnansweredQuestions({ newUnansweredQuestions }, () =>
nextQuestion()
);
I tried run a function after updating the state for unanswered questions but won't work cause it doesn't update right away.
I searched a bit and it is said to use useEffect but I already have one defined and won't let me create another. I just want to call the function nextQuestion after updating UnansweredQuestions
useEffect(() => {
setUnansweredQuestions(questions);
selectRandomQuestion();
}, [currentQuestion]);
There's nothing wrong with having multiple useEffects.
Since you do setUnansweredQuestions and want to run something after that state variable changes, just do:
useEffect(nextQuestion, unansweredQuestions);
You can try like this, I know this is not a good solution, but it works
setState(UnansweredQuestions);
setTimeOut(() => {
nextQuestion()
}, 16)