I am working on a history component in my React project. I have the history state and a setter function setHistory() . I have two functions for handling clicks: handleClick() and removeHistoryItem(), which are called with different buttons. In both of them, I am changing the state with the setHistory() function, but it doesn't work in the handleClick() one.
const removeHistoryItem = () => {
const newHistory = history.filter(item => item.id != id)
setHistory(newHistory)
}
const handleClick = () => {
let newHistory = history
let [selectedGame] = history.filter(item => item.id == id)
const index = history.indexOf(selectedGame)
selectedGame.isFavorite = !selectedGame.isFavorite
newHistory.splice(index, 1, selectedGame)
localStorage.setItem('history', JSON.stringify(newHistory))
setHistory(newHistory)
}
The functions are next to each other so there shouldn't be a problem with the scope. Both functions are executing the other operations well (tested with console.log(), the localStorage is changed just fine, the splice and filter functions are working also). Even the value I am passing to the setHistory() function is correct, just the state is not changed. It's like the setHistory() function is not even called. I am not getting any errors, what could the problem be?
Error is due to Reference equality, both newHistory and history in your handleClick function are referencing same array in "memory", which you mutate with splice, but it is still the same object, react will not detect the change and will not fire the rerender, just because oldValue (history) === newValue (newHistory).
So you need to "recreate" the array in this case. For removeHistoryItem all is ok due to .filter returns the new array already, so new reference, new object.
setHistory([...newHistory]);
Related
I've tried almost every solution similar to my problem, yet none is working. I have a simple state and changing the value of this state in a function as like below, handleOnClick is calling in a button's onClick event. I'm also using Router(if it's change something);
import { useState} from "react"
import { BrowserRouter as Router, Route, Link, useHistory} from "react-router-dom";
const Buton = () => {
let x = "";
const [lowerState, setLower] = useState("")
const history = useHistory();
const handleOnClick = () => {
x = document.getElementById("my_input").value.toLowerCase();
setLower(x)
console.log(x) //this prints the current value
console.log(lowerState) //this DOES NOT prints the current value, but
// when I put another text into the input and click
// to button, it prints the first value I put here
history.push('/test', {params : lowerState})
};
.
.
.
return (...)
}
export default Buton
Now x is a value that returns from an input HTML element. When I set this value as a state and console log, it doesn't print the value first, when I put something in input again, then it prints the first value. So it's like it's coming 1 step behind.
I've used useEffect() , I did put a second parameter to setLower as console.log(lowerState) and other things on the internet that people suggested, but none is working. Every time, the state is coming 1 step behind. How can I make this state changes immediately?
If you want to use the value of an input in a user event function, the best way (and least buggy) is to bind your input value to local state and then just reference that state in your callback function.
Please try to avoid imperatively pulling values from the DOM using getElementById etc. Here's what I mean:
const [value, setValue] = useState('');
// This will keep everything updated until you need to use it
handleChange(event) {
setValue(event.target.value);
}
// Then just grab whatever is in local state
handleClick() {
history.push('/test', {params : value});
}
return (
<input value={value} onChange={handleChange} />
// Your button is here too
)
This is because when you call setLower(x) it is not an async call. So it doesn't wait. That's why you get the 1 step before value in your state right after setting the value.
Official doc - https://reactjs.org/docs/state-and-lifecycle.html#state-updates-may-be-asynchronous
When you call setLower(x), it doesn't immediately update the lowerState. The new value will be available the next time it renders. Because of that the console.log(x) "works" (since it uses the new value that you gain as a parameter) but console.log(lowerState) uses the old value that hasn't updated to the state yet at that point.
If you want history.push('/test', {params : lowerState}) to work, then you need to use the x there instead of lowerState. Or call it within a useEffect with the lowerState and having lowerState as a dependency parameter to the hook.
This is expected behaviour since React is updating state in a batch
Which mean that the state only gets an update after an eventHandler/function is finished
If you want to do some condition, wrap your logic inside a useEffect
useEffect(() => {
if (lowerState === "your-condition-value") {
history.push("/test", { params: lowerState });
}
}, [lowerState]);
Or in your case, just use the variable directly:
const handleOnClick = () => {
x = document.getElementById("my_input").value.toLowerCase();
history.push("/test", { params: x });
};
You should not worry about that since your app still working as expected
So i would like to suggest that use useRef if need for reference only object which may not causing rerendering. also using let x= "" is not correct, you should write code immutable way
const Buton = () => {
const lowerCaseRef = useRef("")
const history = useHistory();
const handleOnClick = () => {
lowerCaseRef.current =
document.querySelector("#my_input").value.toLowerCase();
console.log(lowerCaseRef.current) //this DOES NOT prints the current value, but
// when I put another text into the input and click
// to button, it prints the first value I put here
history.push('/test', {params : lowerCaseRef.current})
};
return (...)
}
I am using useEffect in react to listen to redux(easy-peasy) state change, but I want to listen to 1st value change only.
Because when my page loads the state has a default value and then API call is made and hence data changes but the API is a polling API, hence it keeps getting the data again and again in a short interval of time. But one of my requirement is to listen only to the 1st API data.
This is what I tried:
1st Approach with empty dependency
useEffect(() => {
// my code
},[])
In this case, I get the default value of my state and not the 1st API response.
2nd Approach with state property in the dependency
useEffect(() => {
// my code
},[myState])
In this case, I keep getting the updated response from the API
both of these approaches didn't work for me. So please suggest a better solution.
You can do so using a ref variable and comparing the state with initial state (which could be null, undefined, empty object depending on your implementation):
const hasRun = useRef(false)
useEffect(() => {
if (!hasRun.current && myState !== initialState) {
hasRun.current = true
// run my code
}
},[myState])
A ref variable won't participate in re-rendering.
What I usually do with this is to have a "previous" state.
You can use this hook for to do that one:
const usePrevious(value) {
const ref = useRef();
useEffect(() => {
ref.current = value;
}, [value]); // only re-run if value changes
// return previous value (happens before update in useEffect)
return ref.current;
}
You can then do:
// I usually create a distinct initial state
const [var, setVar] = useState(null);
const prevVar = usePrevious(var);
useEffect(() => {
if (var !== prevVar && var !== null) {
// logic here
}
}, [prevVar, var]);
Yers you can simplify this by not using the usePrevious hook, but the hook is quite handy and can be used to check the previous value and the current one.
I'm using React.js with hooks and trying to change a nested state value.
Since I don't want to mutate the state directly, I'm using lodash's "CloneDeep" function.
The state changes successfully, but a re-render doesn't happen so I don't see the change in the page until I re-enter it.
More info (checks results in my web console):
console.log(logicalLinesDeepCopy == logicalLines); => false
console.log(logicalLinesDeepCopy === logicalLines); => false
console.log(logicalLines[menuLineId].content[menuSentenceId].triggers); => []
console.log(logicalLinesDeepCopy[menuLineId].content[menuSentenceId].triggers); => [(object)]
My code:
// relevant imports:
import CloneDeep from "lodash/cloneDeep";
// relevant state initiation: (the value is filled before I call the relevant function)
const [logicalLines, setLogicalLines] = useState(null);
// relevant function:
const addTriggerToListInState = (trigger) => {
// Deep copy:
let logicalLinesDeepCopy = CloneDeep(logicalLines);
// Change the deep copy:
const lineCopy = logicalLinesDeepCopy[menuLineId];
const triggersListCopy = lineCopy.content[menuSentenceId].triggers;
triggersListCopy.push(trigger);
// Some debug checks:
console.log(logicalLinesDeepCopy == logicalLines);
console.log(logicalLinesDeepCopy === logicalLines);
console.log(logicalLines[menuLineId].content[menuSentenceId].triggers);
console.log(logicalLinesDeepCopy[menuLineId].content[menuSentenceId].triggers);
// Setting the state:
setLogicalLines(logicalLinesDeepCopy);
};
So it looks like both the reference and the nested content of the new state are different from the previous one, then why doesn't it cause a re-render and how can I change it to cause it?
Thanks!
Sapir
I have a functional component that keeps a state. This state I try to manipulate using an onClick event in an SVG. The SVG is in another component and has the addAndRemoveSelectedCabin method passed to it via props. I loop through the elements in an useEffect and add an eventListener. This doesn't work. The useEffect with the selectedCabins dependency logs the new number only. It seems the state returns to the initial state after every stateChange.
This is the state and method in the parent component.
const [selectedCabins, setSelectedCabins] = useState([0]);
const addRemoveSelectedCabin = id => {
const newArr = [...selectedCabins, id];
setSelectedCabins(newArr);
}
useEffect(() => {
console.log(selectedCabins);
}, [selectedCabins])
This is how I call the method. [UPDATE]
useEffect(() =>
{
const cabins = document.querySelectorAll(".cabin");
cabins.forEach(cabin =>
{
const id = cabin.getAttributeNS(null, "id").substring(1, 5);
const found = cabinsData.find(el => el.id === id)
if (found && found.status === "available")
{
cabin.classList.add("green")
cabin.addEventListener('click', () => addRemoveSelectedCabin(id));
} else if (found && found.status === "booked")
{
cabin.classList.add("gray")
}
})
}, [])
Console:
[0]
(2) [0, "1105"]
(2) [0, "1101"]
This works if I put the onClick directly in the SVG element. Does anyone know why this is?
<rect
id="C1105"
x="749.4"
y="58.3"
className="cabin"
width="36.4"
height="19.9"
onClick={() => addRemoveSelectedCabin(1105)}
>
<title>1105</title>
</rect>
As I said in my comment, you are binding addRemoveSelectedCabin in the first render. useEffect is only executed once since you pass an empty dependency list. addRemoveSelectedCabin closes over selectedCabins which at that point in time has the value [0].
Why am I seeing stale props or state inside my function? from the React documentation has more information about this.
The solution in your case is simple: Pass a function to the setter to get the "current" state value. Don't reference the state value in the component:
const addRemoveSelectedCabin = id => {
setSelectedCabins(selectedCabins => [...selectedCabins, id]);
}
Having said that, this is still an odd thing to do in React world. You should reevaluate your assumptions that make you think you have to do it that way.
It's not all the elements that should have a click listener.
Depending on how you actually render the elements, that's easy to do. JSX/React is just JavaScript. Whether you have a condition that adds the event handler or not or whether you have a condition that sets onClick or not is basically the same.
But without a more complete example there is not much we can suggest.
I am trying to filter an array with a string that is input by user. The results are not updating properly with the first key input, then if the box is cleared or characters removed/changed, results that may now pass the filter are not being displayed.
The goal is to have all results displayed on initial page render, then properly updated with each keystroke.
Apologies; I'm just learning to code. Thanks for all assistance.
searchCompUsers = () => {
const newState = {}
const filteredEmps = this.props.employees.filter(
user => user.name.includes(this.state.searchName)
)
console.log(filteredEmps)
`` newState.filterEmps = filteredEmps
this.setState(newState)
}
empSearch = evt => {
const stateToChange = {};
stateToChange[evt.target.id] = evt.target.value;
this.setState(stateToChange);
this.searchCompUsers()
};
These lines are being run in sequence:
this.setState(stateToChange);
this.searchCompUsers();
...
const filteredEmps = this.props.employees.filter(
user => user.name.includes(this.state.searchName)
)
...
this.setState(newState);
I am assuming in your example, evt.target.id is searchName.
Two things you're doing here which you shouldn't do:
Running two setStates in sequence. This isn't necessarily a problem, but there's generally no reason for it and it could mean your code is structured poorly.
Referencing the state immediately after setState. setState is run asynchronously, so you can't guarantee the state will be updated by the time you reach your filter.
The weird results you're getting are probably stemming from (2).
Something like this would work better, assuming the rest of your code is fine:
empSearch = evt => {
const key = evt.target.id;
const value = evt.target.value;
if (key === "searchName") {
const filteredEmps = this.props.employees.filter(
user => user.name.includes(value);
);
this.setState({
filterEmps: filteredEmps
});
}
};
This way, you're only calling setState once per event, and you're not relying on the results of an earlier setState.
If you need to keep searchName in the state for some reason (such as using a controlled component), then you can simply add it to the same setState.
this.setState({
filterEmps: filteredEmps,
searchName: value
});
The only places you can assume the state is up-to-date is in the render() function, and in certain React lifecycle functions. You can also provide a callback to setState if necessary, though this should be relatively rare: this.setState({ ...someState }, () => { ...someCodeToRun() });