<Select
onChange={evt => myFunction('KEY', ['ARRAY', 'OF', 'VALUES'])(evt)}
...
const myFunction = (key, funValues) => {
return (evt: React.ChangeEvent<HTMLSelectElement>) => {
const { values } = evt.target;
if (funValues.find( some condition) ){
callAPI(key, funValues);
}
else{
callAPI(key, values);
}
};
};
I would have written this simply as
onChange={evt => myFunction('KEY', ['ARRAY', 'OF', 'VALUES'], evt)}
I am really failing to see what was the logic of applying currying here, and how it will make
a) this operation better
and/or
b) contributes to broader benefits
Maybe this helps with context, but myFunction is called throughout the codebase. Sometimes via an evt, and sometimes manually.
Creating a curried function (I guess) is a misunderstanding for avoiding an arrow function in the callback handler. In order to this misunderstanding some people define this function as a curried one and use such as:
onChange={myFunction('KEY', ['ARRAY', 'OF', 'VALUES'])}
As you can see this is somehow shorter than your version and works. So, you don't have to use an arrow function and invoke it as you do. event is passed again.
Some people think that this avoids a recreation of this function in each render, but that is not true.
Related
export default function Raffle(){
/*......*/
setCandidates(() => {
candidates.splice(winnerIdx,1) //(*)
return [...candidates];
})
}
}
at (*) line, setCandidates Function remove 2 candidates. but below is run as expected.
setCandidates(() => {
let remain = [...candidates];
remain.splice(winnerIdx,1);
return [...remain];
})
I don't know why the splice method activate twice in first case.
what happened when i modified state in setState function?
You should provide a callback to your set state call, and then copy the argument, not mutate it. Mutating state is generally not good.
setCandidates((oldCandidates) => {
let remain = [...oldCandidates];
remain.splice(winnerIdx,1);
return [...remain];
})
I'm not sure why exactly you were having the problem you were having, but dealing with state this way is much safer and will avoid any weird behavior.
This code seems to work. Nobody does this. What exactly is wrong with it? Under what conditions will it produce bugs?
function App(){
const [b1, n1] = button()
const [b2, n2] = button()
const [input1, text] = input()
return <div>
<div>Total: {n1}+{n2}={n1+n2}. That's {text}.</div>
<div>{b1} {b2} {input1}</div>
</div>;
}
function button() {
const [n, setN] = React.useState(0)
const b = <button onClick={()=>setN(n=>n+1)}>Increment</button>
return [b, n]
}
function input() {
const [t, setT] = React.useState("okay")
const i = <input type="text" value={t} onChange={e=>setT(e.target.value)}/>
return [i, t]
}
https://jscomplete.com/playground/s752284
It's tidy, so I would like to use it if it's fine.
Edit to clarify: Suppose you need to elevate state from child component to parent. There are many ways. Pass down a ref, pass down setState, pass down dispatch, etc. This method doesn't require any passing down, just passing up. Custom hooks typically don't return JSX for some reason. Is there some hidden problem with this pattern?
Edit 2: Extended example
None. It's totally fine. It has even a name, custom hooks: https://reactjs.org/docs/hooks-custom.html
One of the convention is to give it a name that starts with use. That also mean that you need to follow all the rules associated with hooks. Documentation here: https://reactjs.org/docs/hooks-intro.html
Instead of using state directly, why you trying to get it from a function call ? Why can't you create it directly inside your component which is really more readable. And keep this complete part <button onClick={()=>setN(n=>n+1)}>Increment</button> inside the component instead of the way it's given. It's better to write your code in a more readable manner. Otherwise, it becomes too complicated when it comes to much bigger components.
On button pressed my App calls the handlePress function, which returns random objects from an array.
handlePress function:
handlePress = () => {
this.setState({
vitamin: getRandomIngredient(vitaminArray),
}, ()=> matchMealPreference())
}
If I replace matchMealPreference() with a console.log() it works just fine.
After setting the new states I want to call another function immediately with a fat arrow. Otherwise I run into async problems.
The matchMealPreference function looks like this:
matchMealPreference = () => {
if(this.props.mealPreference === this.state.protein.getIngredientFlag()){
return state
} else {
handlePress()
}
}
The function to get a random object from an array:
function getRandomIngredient (arr){
if (arr && arr.length) {
return arr[Math.floor(Math.random() * arr.length)];
}
}
I get the error:
reference error. matchMealPreference is not defined
I'm almost certain that I'm missing something trivial. But I've been stuck at that problem for over a day now, so I thought I'd turn to you for help.
Is there another way to call the matchMealPrefence without the asynchronous problems occuring?
Is it at all possible to call a function at the position where matchMealPreference is called?
Is it unwise to call the handlePress function within the matchMealPrefence function again?
Any help is much appreciated
Edit: Vijay Menon's answer was correct. I needed to add 'this'. Thanks!
You have to reference "matchMealPreference" with "this" keyword inside setState. You would have to do the same for calling "handlePress" inside "matchMealPreference" function
https://codesandbox.io/s/542919430l
changeStr = () => {
this.setState({str:"Changed"})
}
click = () => {
this.setState({num:this.state.num+1},() => {
this.changeStr()
})
}
I'd like to know how does React "freezes" the closure while using the useCallback hook (and with others as well), and then only updates variables used inside the hook when you pass them into the inputs parameter.
I understand that the "freeze" may not be very clear, so I created a REPL.it that shows what I mean: https://repl.it/repls/RudeMintcreamShoutcast. Once you open the code, open your web browser console and start clicking on the count button.
How come the value outside compared to the one inside, for the same variable, is different, if they're under the same closure and referencing the same thing? I'm not familiar with React codebase and so I suppose I'm missing an under the hood implementation detail here, but I tried to think how that could work for several minutes but couldn't come up with a good understanding on how React is achieving that.
The first time the component is rendered, the useCallback hook will take the function that is passed as its argument and stores it behind the scenes. When you call the callback, it will call your function. So far, so good.
The second time that the component is rendered, the useCallback hook will check the dependencies you passed in. If they have not changed, the function you pass in is totally ignored! When you call the callback, it will call the function you passed in on the first render, which still references the same values from that point in time. This has nothing to do with the values you passed in as dependencies - it's just normal JavaScript closures!
When the dependencies change, the useCallback hook will take the function you pass in and replace the function it has stored. When you call the callback, it will call the new version of the function.
So in other words, there's no "frozen"/conditionally updated variables - it's just storing a function and then re-using it, nothing more fancy than that :)
EDIT: Here's an example that demonstrates what's going on in pure JavaScript:
// React has some component-local storage that it tracks behind the scenes.
// useState and useCallback both hook into this.
//
// Imagine there's a 'storage' variable for every instance of your
// component.
const storage = {};
function useState(init) {
if (storage.data === undefined) {
storage.data = init;
}
return [storage.data, (value) => storage.data = value];
}
function useCallback(fn) {
// The real version would check dependencies here, but since our callback
// should only update on the first render, this will suffice.
if (storage.callback === undefined) {
storage.callback = fn;
}
return storage.callback;
}
function MyComponent() {
const [data, setData] = useState(0);
const callback = useCallback(() => data);
// Rather than outputting DOM, we'll just log.
console.log("data:", data);
console.log("callback:", callback());
return {
increase: () => setData(data + 1)
}
}
let instance = MyComponent(); // Let's 'render' our component...
instance.increase(); // This would trigger a re-render, so we call our component again...
instance = MyComponent();
instance.increase(); // and again...
instance = MyComponent();
I came here with a similar, rather vague uncertainty about the way useCallback works, its interaction with closures, and the way they are "frozen" by it. I'd like to expand a bit on the accepted answer by proposing to look at the following setup, which shows the working of useCallback (the important aspect is to ignore the linter's warning, for pedagogical reasons):
function App() {
const [a, setA] = useState(0)
const incrementWithUseCallback = useCallback(() => {
// As it closes on the first time `App` is called, the closure is "frozen" in an environment where a=0, forever
console.log(a)
setA(a + 1)
}, []) // but.. the linter should complain about this, saying that `a` should be included!
const incrementWithoutUseCallback = () => {
// This will see every value of a, as a new closure is created at every render (i.e. every time `App` is called)
console.log(a)
setA(a + 1)
}
return (
<div>
<button onClick={incrementWithUseCallback}>Increment with useCallback</button>
<button onClick={incrementWithoutUseCallback}>Increment without useCallback</button>
</div>
)
}
So we clearly see that useCallback effectively "freezes" its closure at a certain moment in time, which is a concept that must be understood clearly, in order to avoid confusing problems, which are sometimes also referred as "stale closures". This article probably does a better job of explaining it than me: https://tkdodo.eu/blog/hooks-dependencies-and-stale-closures
Here's a slightly another view on example code provided by Joe Clay, which emphasizes closure context in which callback is called.
//internal store for states and callbacks
let Store = { data: "+", callback: null };
function functionalComponent(uniqClosureName) {
const data = Store.data;//save value from store to closure variable
const callback = Store.callback = Store.callback || (() => {
console.log('Callback executed in ' + uniqClosureName + ' context');
return data;
});
console.log("data:", data, "callback():", callback());
return {
increase: () => Store.data = Store.data + "+"
}
}
let instance = functionalComponent('First render');
instance.increase();
instance = functionalComponent('Second render');
instance.increase();
instance = functionalComponent('Third render');
As you see, callback without dependencies will be always executed in the closure where it was memorized by useCallback, thus 'freezing' closure.
It happens because when function for callback is created, it is created only once, during first 'render'. Later this function is re-used, and use value of data which was recorded from Store.data during first call.
In the next example you can see the closure 'freezing' logic "in essence".
let globalX = 1;
const f = (() => {
let localX = globalX; return () => console.log(localX); }
)();
globalX = 2;//does not affect localX, it is already saved in the closure
f();//prints 1
The Norm
According to the official documentation (https://github.com/gaearon/redux-thunk), I know that redux thunk allows dispatching a chain of async actions sequentially like so:
function makeSandwichesForEverybody() {
return function (dispatch, getState) {
return dispatch(
makeASandwichWithSecretSauce('My Grandma')
).then(() =>
Promise.all([
dispatch(makeASandwichWithSecretSauce('Me')),
dispatch(makeASandwichWithSecretSauce('My wife'))
])
).then(() =>
dispatch(makeASandwichWithSecretSauce('Our kids'))
).then(() =>
dispatch(getState().myMoney > 42 ?
withdrawMoney(42) :
apologize('Me', 'The Sandwich Shop')
)
);
}
}
My Situation
However, what if I have a dynamic array of actions that I want to iterate through and call?
let arrOfActions = [];
arrOfActions.push(action1);
arrOfActions.push(action2);
arrOfActions.push(action3);
How can I chain these async actions iteratively using Promise logic? To best explain what I am thinking, I am hoping to do something like this:
function thunkActionCreator() {
return function (dispatch, getState) {
for (let key of arrOfActions) {
dispatch(arrOfActions[key]()).then(
// run next action in arrOfActions here
)
}
}
}
Is this dynamic iteration of function calls possible? If so, what is the syntax?
For verification that you can indeed call functions in an array of functions, here is the resource I found: How to store functions into an array and loop through each in javascript
Why A Dynamic Array of Actions?
There may be a better way to think of this, but the reason I am trying to use this implementation is because I have a series of functions that I need to call in a specific order. This array would be stored in Redux's store, and I am not sure how else to be able to call a series of functions in order from start to finish. Any other ideas would be helpful!
Upfront disclaimer; I think the fact that you need to do this is evidence of deeper problems in your codebase. You really shouldn't be queueing up a list of async functions that need to occur in a specific order and which you don't know of in advanced. That's a number of red flags.
But can you do it? Sure!
function enqueueDynamicArray(functionArray) {
let p = Promise.resolve();
for(index in functionArray) {
p = p.then(functionArray[index]);
}
return p;
}
EDIT: And per comments, if you can rely on the functions being synchronous;
function callDynamicArray(functionArray) {
for(index in functionArray){
functionArray[index]();
};
}