How do I memoize the creation of JSX elements with useMemo? - javascript

I have a function which essentially acts as a .map to create JSX elements out of a list. I wanted to verify that my chain of logic for the memoization is correct, and that I have understood the dependencies.
const createJSXButtons = useCallback((buttonList) => {
const jsxList = [];
for (let button of buttonList) {
jsxList.push(<button isToggled={parentState}>{button}</button>);
}
return jsxList;
}, [parentState]);
This way, when I execute something like:
const myButtons = createJSXButtons(['R', 'U', 'F']);
I would get 3 JSX buttons inside myButtons.
As you can see, createJSXButtons captures a parent state within a closure, and is added as a dependency to the useCallback to prevent stale data.
However, I would also like to memoize the creation of myButtons, how can I do this? My first instinct was to use useMemo, but I was hoping someone could verify my approach.
const myButtons = useMemo(() => {
createJSXButtons(['R', 'U', 'F'])
}, [createJSXButtons]);
The idea is whenever createJSXButtons changes, which only happens when parentState changes, then and only then myButtons will change. And we need myButtons to change when the parentState changes to ensure that each button is correctly receiving the correct isToggled boolean.
Have I understood the above flow of logic correctly? Kindly note I'm not asking how to use useMemo, but rather if my chain of logic is correct, specific to this use case.

Related

Why does React/Typescript complain want me to use my return function in useEffect dependency array?

I'm making a checkbox component for my project, and I have it set so that whenever is isChecked state changes, useEffect will run a returnFunction (defined in the props), like so:
export function JCheckbox({ returnFunction }: JCheckboxProps) {
const [isChecked, setIsChecked] = React.useState(defaultState)
React.useEffect(() => {
returnFunction(isChecked)
}, [isChecked])
When I do this, my compiler wants me to include returnFunction in the useEffect dependency array. I'm confused why I would need to track that, it's not used at all anywhere else in the function, and it doesn't change either. The code works perfectly fine without it.
You should add your function (returnFunction) to useEffect dependency because your function reference (returnFunction) may be changed, So react ask you to add your function (returnFunction) in useEffect dependency to recognize that and update them by new function reference.
returnFunction is certainly used as it is called inside the effect, and it could certainly change as the parent component could pass in different props, in which case your component would continue to call the previously passed in function. So the linter is correct, returnFunction is a dependency. However adding it makes it slightly more difficult to detect changes of isChecked, as changing returnFunction would also trigger the effect. Thus it seems way more elegant to call returnFunction from within setIsChecked:
const [isChecked, _setIsChecked] = useState(false);
function setIsChecked(value: boolean) {
returnFunction(value);
_setIsChecked(value);
}
However in the case given it seems that the state should rather be maintained by the parent component, consider lifting it up.
returnFunction is read from props so you probably must include it in the array of dependencies.
You should almost always include every variable and function you declare inside React and use in useEffect callback in the array of dependencies.
Eslint does not expect you to include setState functions and refs that you create in the same component that you execute useEffect.
Edit:
If you have a handleChange event handler and isChecked is toggled whenever the onChange method is invoked, you probably do not need useEffect. Call returnFunction function with the new isChecked value.

Children useCallback dependency hell

From what I understand you use useCallback to prevent rerendering so I've been using it in every function and my spider senses are telling me it already sounds bad.
But the story doesn't ends there, since I've been using it everywhere I'm now passing dependencies to all my child components that they shouldn't need to worry about like in the following example :
EDIT // SANDBOX: https://codesandbox.io/s/bold-noether-0wdnp?file=/src/App.js
Parent component (needs colorButtons and currentColor)
const ColorPicker = ({onChange}) => {
const [currentColor, setCurrentColor] = useState({r: 255, g:0, b: 0})
const [colorButtons, setColorButtons] = useState({0: null})
const handleColorButtons = useCallback((isToggled, id) => {
/* code that uses colorButtons and currentColor */
}, [colorButtons, currentColor])
return <div className="color-picker">
<RgbColorPicker color={currentColor} onChange={setCurrentColor} />
<div className="color-buttons">
{
Object.entries(colorButtons).map(button => <ColorButton
//...
currentColor={currentColor}
onClick={handleColorButtons}
colorButtons={colorButtons}
/>)
}
</div>
</div>
}
1st child (needs style and currentColor but gets colorButtons for free from its parent)
const ColorButton = ({currentColor, onClick, id, colorButtons}) => {
const [style, setStyle] = useState({})
const handleClick = useCallback((isToggled) => {
/* code that uses setStyle and currentColor */
}, [style, currentColor, colorButtons])
return <ToggleButton
//...
onClick={handleClick}
style={style}
dependency1={style}
dependency2={currentColor}
dependency3={colorButtons}
>
</ToggleButton>
}
2nd child (only needs its own variables but gets the whole package)
const ToggleButton = ({children, className, onClick, style, data, id, onRef, ...dependencies}) => {
const [isToggled, setIsToggled] = useState(false)
const [buttonStyle, setButtonStyle] = useState(style)
const handleClick = useCallback(() => {
/* code that uses isToggled, data, id and setButtonStyle */
}, [isToggled, data, id, ...Object.values(dependencies)])
return <button
className={className || "toggle-button"}
onClick={handleClick}
style={buttonStyle || {}}
ref={onRef}
>
{children}
</button>
}
Am I doing an anti-pattern and if so, what is it and how to fix it ? Thanks for helping !
React hook useCallback
useCallback is a hook that can be used in functional React components. A functional component is a function that returns a React component and that runs on every render, which means that everything defined in its body get new referential identities every time. An exception to this can be accomplished with React hooks which may be used inside functional components to interconnect different renders and maintain state. This means that if you save a reference to a regular function defined in a functional component using a ref, and then compare it to the same function in a later render, they will not be the same (the function changes referential identity between renderings):
// Render 1
...
const fnInBody = () => {}
const fn = useRef(null)
console.log(fn.current === fnInBody) // false since fn.current is null
fn.current = fnInBody
...
// Render 2
...
const fnInBody = () => {}
const fn = useRef(null)
console.log(fn.current === fnInBody) // false due to different identity
fn.current = fnInBody
...
As per the docs, useCallback returns "a memoized version of the callback that only changes if one of the dependencies has changed" which is useful "when passing callbacks to optimized child components that rely on reference equality to prevent unnecessary renders".
To sum up, useCallback will return a function that maintains its referential identity (e.g. is memoized) as long as the dependencies don't change. The returned function contains a closure with the used dependencies and must thus be updated once the dependencies change.
This results in this updated version of the previous example
// Render 1
...
const fnInBody = useCallback(() => {}, [])
const fn = useRef(null)
console.log(fn.current === fnInBody) // false since fn.current is null
fn.current = fnInBody
...
// Render 2
...
const fnInBody = useCallback(() => {}, [])
const fn = useRef(null)
console.log(fn.current === fnInBody) // true
fn.current = fnInBody
...
Your use case
Keeping the above description in mind, let's have a look at your use of useCallback.
Case 1: ColorPicker
const handleColorButtons = useCallback((isToggled, id) => {
/* code that uses colorButtons and currentColor */
}, [colorButtons, currentColor])
This function will get a new identity every time colorButtons or currentColor changes. ColorPicker itself rerenders either when one of these two are set or when its prop onChange changes. Both handleColorButtons and the children should be updated when currentColor or colorButtons change. The only time the children benefit from the use of useCallback is when only onChange changes. Given that ColorButton is a lightweight component, and that ColorPicker rerenders mostly due to changes to currentColor and colorButtons, the use of useCallback here seems redundant.
Case 2: ColorButton
const handleClick = useCallback((isToggled) => {
/* code that uses setStyle and currentColor */
}, [style, currentColor, colorButtons])
This is a situation similar to the first case. ColorButton rerenders when currentColor, onClick, id or colorButtons change and the children rerender when handleClick, style, colorButtons or currentColor change. With useCallback in place, the props id and onClick may change without rerendering the children (according to the above visible code at least), all other rerenders of ColorButton will lead to its children rerendering. Again, the child ToggleButton is lightweight and id or onClick are not likely to change more often than any other prop so the use of useCallback seems redundant here as well.
Case 3: ToggleButton
const handleClick = useCallback(() => {
/* code that uses isToggled, data, id and setButtonStyle */
}, [isToggled, data, id, ...Object.values(dependencies)])
This case is elaborate with a lot of dependencies but from what I see, one way or the other, most of the component props will lead to a "new version" of handleClick and with the children being lightweight components, the argument to use useCallback seems weak.
So when should I use useCallback?
As the docs say, use it in the very specific cases when you need a function to have referential equality between renders ...
You have a component with a subset of children that are expensive to rerender and that should rerender much less often than the parent component but rerender due to a function prop changing identity whenever the parent rerenders. To me, this use case also signals bad design and I would attempt to divide the parent component into smaller components but what do I know, maybe this is not always possible.
You have a function in the body of the functional component which is used in another hook (listed as a dependency) which is triggered every time due to the function changing identity whenever the component rerenders. Typically, you can omit such a function from the dependency array by ignoring the lint rule even if this is not by the book. Other suggestions are to place such a function outside the body of the component or inside the hook that uses it, but there might be scenarios where none of this works out as intended.
Good to know connected to this is ...
A function living outside a functional component will always have referential equality between renders.
The setters returned by useState will always have referential equality between renders.
I said in the comments that you can use useCallback when there is function doing expensive calculations in a component that rerenders often but I was a bit off there. Let's say you have a function that does heavy calculations based on some prop that changes less often than a component rerenders. Then you COULD use useCallback and run a function inside it that returns a function with a closure with some computed value
const fn = useCallback(
(
() => {
const a = ... // heavy calculation based on prop c
const b = ... // heavy calculation based on prop c
return () => { console.log(a + b) }
}
)()
, [c])
...
/* fn is used for something, either as a prop OR for something else */
This would effectively avoid calculating a and b every time the component rerenders without c changing, but the more straightforward way to do this would be to instead
const a = useMemo(() => /* calculate and return a */, [c])
const b = useMemo(() => /* calculate and return b */, [c])
const fn = () => console.log(a + b)
so here the use of useCallback just complicates things in a bad way.
Conclusion
It's good to understand more complicated concepts in programming and to be able to use them, but part of the virtue is also to know when to use them. Adding code, and especially code that involves complicated concepts, comes at the price of reduced readability and code that is harder to debug with a lot of different mechanisms that interplay. Therefore, make sure you understand the hooks, but always try to not use them if you can. Especially useCallback, useMemo and React.memo (not a hook but a similar optimization) should, in my opinion, only be introduced when they are absolutely needed. useRef has its very own use cases but should also not be introduced when you can solve your problem without it.
Good work on the sandbox! Always makes it easier to reason about code. I took the liberty of forking your sandbox and refactoring it a bit: sandbox link. You can study the changes yourself if you want. Here is a summary:
It's good that you know and use useRef and useCallback but I was able to remove all the uses, making the code much easier to understand (not only by removing these uses but by also removing the contexts where they are used).
Try to work with React to simplify things. I know this is not a hands-on suggestion but the more you get into React, the more you will realize that you can do things co-operating with React, or you can do things your own way. Both will work but the latter will result in more headache for you and everybody else.
Try to isolate the scope of a component; only delegate data that is necessary to child components and constantly question where you keep your state. Earlier you had click handlers in all three components and the flow was so complicated I didn't even bother to fully understand it. In my version, there is just one click handler in ColorPicker that is being delegated down. The buttons don't have to know what happens when you click them as long as the click handler takes care of that. Closures and the ability to pass functions as arguments are strong advantages of React and Javascript.
Keys are important in React and it's good to see that you use them. Typically, the key should correspond to something that uniquely identifies a specific item. Good to use here would be ${r}_${g}_${b} but then we would only be able to have one sample of each color in the button array. This is a natural limitation but if we don't want it, the only way to assign keys is to assign a unique identifier, which you did. I prefer using Date.now() but some would probably advise against it for some reason. You could use a global variable outside the functional component too if you don't want to use a ref.
Try to do things the functional (immutable) way, and not the "old" Javascript way. For example, when adding to an array, use [...oldArray, newValue] and when assigning to an object, use {...oldObject, newKey: newValue }.
There are more things to say but I think it's better for you to study the refactored version and you can let me know if you wonder about anything.

How to give react components dynamic ids, when React runs code twice?

It's a known React behavior that code runs twice.
However, I'm creating a form builder in which I need to be able to give each form input a dynamic Id and use that Id for a lot of other purposes later. Here's a simple code of an input:
const Text = ({placeholder}) => {
const [id, setId] = useState(Math.random());
eventEmitter.on('global-event', () => {
var field = document.querySelector(`#${id}`); // here, id is changed
});
}
But since Math.random() is a side-effect, it's called twice and I can't create dynamic ids for my form fields.
The reason I'm using document.querySelector can be read here.
My question is, how can I create consistent dynamic ids for my inputs?
It seems you think that useState(Math.random()); is the side-effect causing you issue, but only functions passed to useState are double-invoked.
I think the issue you have is that the eventEmitter.on call is the unintentional side-effect since the function component body is also double invoked.
Strict mode can’t automatically detect side effects for you, but it
can help you spot them by making them a little more deterministic.
This is done by intentionally double-invoking the following functions:
Class component constructor, render, and shouldComponentUpdate methods
Class component static getDerivedStateFromProps method
Function component bodies <-- this
State updater functions (the first argument to setState)
Functions passed to useState, useMemo, or useReducer <-- not this
To remedy this I believe you should place the eventEmitter.on logic into an useEffect hook with a dependency on the id state. You should also probably use id values that are guaranteed a lot more uniqueness. Don't forget to return a cleanup function from the effect to remove any active event "listeners", either when id updates, or when the component unmounts. This is to help clear out any resource leaks (memory, sockets, etc...).
Example:
import { v4 as uuidV4 } from 'uuid';
const Text = ({placeholder}) => {
const [id, setId] = useState(uuidV4());
useEffect(() => {
const handler = () => {
let field = document.querySelector(`#${id}`);
};
eventEmitter.on('global-event', handler);
return () => {
eventEmitter.removeListener('global-event', handler);
};
}, [id]);
...
}

React Hooks: Adding new fields to an Object state does not get reflected immediately

I am using React Hooks to manage states within a component.
const addNode = () => {
let pform = pForm
let handles = [vForm, yForm, hForm]
let access_info = [virtualForm, management1Form, management2Form, consoleForm]
let newObj = {
...currentForm,
p: pform,
handles: handles,
access_info: access_info,
}
console.log('newObj', newObj)
setCurrentForm(
newRouterObj
)
console.log(currentForm)
let currArr = [...addedNodes]
currArr.push(currentForm)
setAddedNodes(currArr)
intializeForms()
}
The function above is an onClick that I use when I press an Add button. The forms (pForm, vForm, yForm, etc.) are all separate states. I gather them together and put them into a single object newObj and use setCurrentForm to update the currentForm state to newObj.
When I console.log the newObj, everything goes in fine. However, when I check the currentForm after the setCurrentForm, the fields (p, handles, and access_info) are empty.
I know that states in React can have a delay in updates so I might have to use useEffect. However, in my use case, which is to gather different states and put them in as a new field in the currentForm state seems useEffect is not the best way to solve it. Can anyone help please?
You are misunderstanding exactly how useState works. When you call the useState setter function, the state value isn't actually updated immediately, instead it will trigger the component to re-render with the updated value. Even though you call the setter half way through the function, the state value will remain the original value for the entire lifetime of that function call.
You could slightly tweak what you have to be
const addNode = () => {
...
let currArr = [...addedNodes]
// you know that currentForm is supposed to be newObj, so just push that
// see my explanation above to understand why it currentForm isn't what you expect
currArr.push(newObj)
...
}
It's an async action so values will not be assigned/updated instantly. You need to watch for the changes using useEffect hook to log new values and to do anything in case
useEffect(() => {
// Whenever `currentForm` will be updated, this callback will be invoked
console.log('updated currentForm values', currentForm);
},[currentForm]);

What is the intention of using React's useCallback hook in place of useEffect?

I'm trying to understand what the use case is for using React's useCallback hook in place of the useEffect hook.
They both appear to act as a listener for state changes of their inputs (examples taken from the React Docs):
useEffect(
() => {
const subscription = props.source.subscribe();
return () => {
subscription.unsubscribe();
};
},
[props.source],
);
const memoizedCallback = useCallback(
() => {
doSomething(a, b);
},
[a, b],
);
But, the useEffect hook gives the additional benefit of cleaning up resources where you would have previously with componentWillUnmount.
So, what is a good use case for using useCallback? And, what am I missing here?
useEffect has very specific timing aspects related to it that you can read about here. The function specified will be executed after rendering is complete and the DOM has been updated. This will happen after each rendering where any of the values specified in the second-argument array change.
useCallback doesn't automatically execute anything. It returns a function that can be executed by whatever code needs to trigger it. There is no listening to changes that causes an execution of the callback. The array values just control what instance of the function is returned. The array values do not control the timing of the function execution.
A key use case is to pass this function as a prop to a child component to use as an event handler. useCallback allows you to define an inline function to use as an event handler (thus it has access to any other variables in the context where the function is defined) without the downside of passing a unique prop to the child every render. So long as the values in the second-argument array have not changed, the same function will be returned as was returned the previous rendering. So if the child component is a pure component, it will not be forced to re-render simply because of always receiving a unique event handler function.
without useCallback
const Parent = ()=> {
const [a, setA] = useState(null);
const eventHandler = ()=> {
// every render creates a unique instance of eventHandler
// even though it always does the same thing so long as 'a' hasn't changed
doSomethingWithA(a);
}
return <Child onClick={eventHandler}/>
}
with useCallback
const Parent = ()=> {
const [a, setA] = useState(null);
const eventHandler = useCallback(()=> {
// A unique function instance is passed in to useCallback on every render, but
// eventHandler will be set to the first instance of this function
// (i.e. potentially an instance of the function that was passed to useCallback
// on a previous rendering) that was passed to useCallback
// for the current value of 'a'.
doSomethingWithA(a);
}, [a]);
return <Child onClick={eventHandler}/>
}
This article provides a bit more detail than the React docs on the use case for useCallback and other hooks.
Related answer: Trouble with simple example of React Hooks useCallback

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