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How can you re-render a React functional component with new props?

I want to do something like this:
const GreetingWithCounter = (props) => {
const { name, count } = props;
return (
<div>
<div>Hello {name}</div>
<button onClick={() => render({ ...props, count: count + 1 })}>
{count}
</button>
</div>
);
}
<GreetingWithCounter name="Alice" count={0} />
Ie. I want to re-render a component with new values for its props. Is there a way to do that? Looking through these three questions, I'm seeing ways to re-render a component but not with new values for props (1, 2, 3).
Context
I'm thinking about a way to simplify React. I really like the mental model of React being the view layer in MVC, where UI = F(state). But things can get confusing when "state" can come from so many different places: props, useState, useReducer, "raw" useContext, Redux (which uses useContext I think), whatever else.
What if everything was just based off of props?
For local state you'd do what I did in that example above. You'd initialize the local state of count when doing <GreetingWithCounter name="Alice" count={0} /> and then update it by re-rendering. This means less DRYness because you'd have to repeat the count={0} code instead of only having it once inside of GreetingWithCounter.
You'd have to do prop drilling instead of useContext stuff.
This approach would probably make React slower.
Still, I hypothesize 1) that the mental model of having everything coming from props is simpler and 2) that pro outweighs the cons in a non-trivial amount of apps.

Props are not supposed to be mutated in React. That is precisely the difference between props and state. The React way to do this is to use state for the count. You can pass the initial state of the count as a prop and do this: const [count, setCount] = useState(initialCount). Your onClick handler would then increment count, which again is state. I realize that this is not what you want but it's how React works.

In React Props values cannot be changed in child component but we can do it in parent component.
const GreetingWithCounter = (props) => {
const { name, count, updateCount } = props;
return (
<div>
<div>Hello {name}</div>
<button onClick={updateCount}>{count}</button>
</div>
);
};
function App() {
const [count, setCount] = useState(0);
const updateCount = () => {
setCount(count + 1);
};
return (
<div className='App'>
<h1>Greeting With Counter:</h1>
<GreetingWithCounter
name='Alice'
count={count}
updateCount={updateCount}
/>
</div>
);
}

Appreciate the change you want to point out and value you want to add but there might be some points that you're missing what React conceptually trying to provide with seperation between props and state.
The props within components coming with React are specifically conceptually designed to be immutable as per the documentation
here.
So what you're trying to do is conceptually not ok for that purpose and violating what React tries to accomplish.
Infact you may mention about creating another library/framework which successfully getting it done while introducing props are the new state concept but in this specific case, there's no possible way to succeed on it in a React way.

You cannot change value of props in child but you have 2 ways to handle it
first, I assume that you only want to use count in child component and you don't need count value in parent, in this case you can use props.count as initial state, sth like this :
const GreetingWithCounter = props => {
const [count, setCount] = useState(props.count);
const { name } = props;
return (
<div>
<div>Hello {name}</div>
<button onClick={() => setCount(prevState => prevState + 1)}>{count}</button>
</div>
);
};
<GreetingWithCounter name="Alice" count={0} />;
but if you wanna access it's value from parent, it's better to pass setter to child
sth like this :
const GreetingWithCounter = ({name,count,setCount}) => {
return (
<div>
<div>Hello {name}</div>
<button onClick={() => setCount(prevState => prevState + 1)}>{count}</button>
</div>
);
};
const App = ()=>{
const [count, setCount] = useState(0);
return (<GreetingWithCounter name="Alice" count={count} setCount={setCount} />)
}
or if it's child is so deep that you need to send props to all it's tree, its better to use state management like Redux,Context or ...

Is this the way you want to do ? :
import React from 'react'
import ReactDOM from 'react-dom'
export default function renderComponent(Component, props, container) {
ReactDOM.render(<Component {...props} />, container)
}

What you are trying to do goes against the philosophy of state management of react. For correct way to do it, you can check other answers, and even you yourself have posted it in the questions.
But if you really want to do it, behind its magic, React is also just JavaScript. Therefore, we just need to implement the render function outside of React way of thinking. We know that React re-renders on state change magic or on props change. We need to just somehow connect the render method you asked for with set state. Something like the below should work.
const ParentStuff = () => {
const [props, setProps] = useState({ name: "Alice", count: 0 });
render = setProps;
return (<GreetingWithCounter name={props.name} count={props.count} />);
}
let render;
const GreetingWithCounter = props => {
const { name, count } = props;
return (
<div>
<div>Hello {name}</div>
<button onClick={() => render({ ...props, count: count + 1 })}>{count}</button>
</div>
);
};
A lot of people will scream though at code above. It definitely strays away from the intended use.
If you want to go further, you can also just have one state for the entire app, and pass this state fo every component. And voila! You just created a singleton state and an uni directional data flow, which is a poor man version of the redux and this will probably kill performance of the webapp, as things like typing each letter in a textbox will re-render the entire page.

As others already mentioned, component is either controlled or uncontrolled (or mix of both) in react.
If you keep state in component itself - it's uncontrolled. You can reset its state to internal by changing key prop from parent though.
If you keep state in parent - it's controlled component and changes it's state through props/callbacks.
What you have shown in your example, you want to achieve uncontrolled component with some syntactic sugar on top.
Example implementation:
const usePropsWithRender = (props) => {
const [currentProps, setCurrentProps] = useState(props);
return {
...currentProps,
render: setCurrentProps,
};
};
const GreetingWithCounter = (props) => {
const { name, count, render } = usePropsWithRender(props);
return (
<div>
<div>Hello {name}</div>
<button onClick={() => render({ ...props, count: count + 1 })}>
{count}
</button>
</div>
);
};
You can reuse usePropsWithRender through all you project, but it's nothing more than a thin wrapper around useState. I don't see how it is better than using useState directly.

react testing library ternary operator, finding right component

I'm beginner with React testing, learning by coding, here i have a component 'cam.tsx'
i want to test it, when i want to test Add function it goes straight like this, but when i want to test Update function it still shows Add function in my test, how to test both of them ?
Add and Update functions are forms where user can fill.
describe("Testing component ", () => {
const Camera = (): RenderResult =>
render(
<Provider store={store}>
<Cam
}}
/>{" "}
</Provider>
);
test("Cam", () => {
Camera();
const name = screen.queryByTestId(/^AddName/i);
});
});
cam.tsx:
const ADD = "ADD";
let [state, setState] = useState<State>({mode: ADD });
if (props.mode) {
state.mode = props.mode;
}
const option = state.mode;
return (
<React.Fragment>
<div data-testid="header">
{option == ADD ? Add() : <></>}
{option == UPDATE ? Update() : <></>}
</div>
</React.Fragment>
Basically cam.tsx is a component which has two forms one for updating camera and another for adding new camera.When user clicks add/update icon then cam component gets 'mode' via props ' state.mode = props.mode '
English is not my mother language, so could be mistakes

Here is how to test a component that conditionally renders components from state and can be updated via props.
import {render, screen} from '#testing-library/react';
import {Cam} from './Cam';
test('renders add by default', () => {
render(<Cam/>);
expect(screen.getByTestId('addForm'))
.toBeInTheDocument();
expect(screen.queryByTestId('updateForm'))
.not.toBeInTheDocument();
});
test('renders edit by passing props', () => {
const {rerender} = render(<Cam mode={undefined}/>);
rerender(<Cam mode={'UPDATE'} />)
expect(screen.getByTestId('updateForm'))
.toBeInTheDocument();
expect(screen.queryByTestId('addForm'))
.not.toBeInTheDocument();
});
However, it is known in the React community that updating state via props is usually an anti-pattern. This is because you now have two sources of truth for state and can be easy to have these two states conflicting. You should instead just use props to manage rendering.
If state comes from a parent component, use props.
export function Cam(props) {
const option = props.mode;
return (
<div data-testid="header">
{option === ADD ? Add() : <></>}
{option === UPDATE ? Update() : <></>}
</div>
);
}
If you really want to keep state in the child component even if props are passed in, you should update props in an useEffect hook. Additionally, you should use the setState function rather than setting state manually state.mode = props.mode
Use the useEffect hook to update state via props.
...
const [state, setState] = useState({mode: ADD});
useEffect(() => {
if (props.mode) {
setState({mode: props.mode});
}
}, [props.mode]) <-- checks this value to prevent infinite loop.
const option = state.mode;
return (
...

state neither updated from useState nor useEffect inside useState [duplicate]

This question already has answers here:
The useState set method is not reflecting a change immediately
(15 answers)
Closed 2 years ago.
I was facing almost the same problem as in this question
The code is a bit too much so I have made a stripped down version of the problem. (please forgive me if I made a mistake in doing so)
Basically, I have a main component and a sub component
main component
import React {useState} from 'react'
import SubComponent from './subcomponent';
const Main = (props) => {
const [state, setState] = useState(null);
const updateStateFunction = (data) => {
const newState = data
console.log(state)
setState(newState)
console.log(state)
}
return (
<div>
<SubComponent
updateStateFunction = {updateStateFunction}
/>
</div>
)
}
export default Main;
sub component
import React {useState} from 'react'
const SubComponent = ({updateStateFunction}) => {
return (
<div>
<button
onClick={() => updateStateFunction("Something new")}
>
</button>
</div>
)
}
export default SubComponent;
both the console logs give null.
My attempts at a solution:
Since most stack overflow answers suggested that stateupdates using hooks is asynchronous I tried using setTimeout
I thought we could then use async-await but that was a wrong approach
I tried updating the state inside useEffect but the point is that nothing is being re redered. This is because the variable that is being updated is never a part of an output but rather sort a helper varibale for further operations.
The way I did this was using the solution in the above refereced question:
const Main = (props) => {
/*Debugging*/
let newState = null
useEffect(()=>{
console.log("useEffect called")
setState(newState)
}, [newState])
/*Debugging*/
const [state, setState] = useState(null);
const updateStateFunction = (data) => {
newState = data
console.log(state)
setState(newState)
console.log(state)
}
return (
<div>
<SubComponent
updateStateFunction = {updateStateFunction}
/>
</div>
)
}
I though since the useEffect hook is not even being executed hence I did not try the other two methods in the solution
Am I referencing the wrong type of problem or is this a common behaviour in react?
Happy to provide any more information if needed
Edit:
I have added console.log() because I have operations followed by the state change that uses the value of the state variable.
Using React dev tools I see that the state is updating and that too almost instantly. The problem is that the button press leads to a dialogue pop-up in the real code whose component uses the state for other logic, hence I get an error that that state is still null

I am not sure how let newState = null has anything to do with any of the answers in the quoted question, so to be clear, this is how one would directly apply the accepted answer:
const Main = (props) => {
const [state, setState] = useState(null);
const updateStateFunction = (data) => { setState(data) }
useEffect(() => { console.log(state) }, [state])
return <SubComponent updateStateFunction = {updateStateFunction} />
}
However, there is no point of changing a state if it's not used for anything rendered on the screen - if Reacts does not detect any change in the return value, it might not commit any change to the DOM as a part of the optimizations, so it would probably not run any useEffects in that case.
I would recommend using React Dev Tools for checking the current state of a Component.
Also, console.log is basically the only side effect which does not need to be inside useEffect. If directly in the render function, it would be executed whenever the render function is called and would not depend on React committing changes to DOM...
Note that the first advice in my answer to the quoted question does not wrap console.log into useEffect - and to be clear again, this is how one would directly apply that advice:
const Main = (props) => {
const [state, setState] = useState(null);
const updateStateFunction = (data) => { setState(data) }
console.log(state)
return <SubComponent updateStateFunction = {updateStateFunction} />
}

The setting of the state is asynchronous in react.
An asynchronous function will be executed in parallel (well, kind of) as other instructions. Rather than console.logging after setting state, you could console.log state before the return function to know the new output.
There is nothing wrong with your first implementation. No need to use useEffect here
import React {useState} from 'react'
import SubComponent from './subcomponent';
const Main = (props) => {
const [state, setState] = useState(null);
const updateStateFunction = (data) => {
setState(data)
}
// try console logging here
console.log(state)
return (
<div>
<SubComponent
updateStateFunction = {updateStateFunction}
/>
</div>
)
}
export default Main;
Think of it like this, whenever the state is set, your function which contains the state gets refreshed and re-run. You could console.log anywhere in the function to get the new state.

Use the react devtools to debug, even if the console.log() display null you see the state change in the devtools. The reason is that the state update is asynchronous.
If you still want to debug your react app using console.log() then call it just before the return statement or even in the jsx directly using curly braces.

Is it possible to share states between components using the useState() hook in React?

I was experimenting with the new Hook feature in React. Considering I have the following two components (using React Hooks) -
const HookComponent = () => {
const [username, setUsername] = useState('Abrar');
const [count, setState] = useState();
const handleChange = (e) => {
setUsername(e.target.value);
}
return (
<div>
<input name="userName" value={username} onChange={handleChange}/>
<p>{username}</p>
<p>From HookComponent: {count}</p>
</div>
)
}
const HookComponent2 = () => {
const [count, setCount] = useState(999);
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
Hooks claim to solve the problem of sharing stateful logic between components but I found that the states between HookComponent and HookComponent2 are not sharable. For example the change of count in HookComponent2 does not render a change in the HookComponent.
Is it possible to share states between components using the useState() hook?

If you are referring to component state, then hooks will not help you share it between components. Component state is local to the component. If your state lives in context, then useContext hook would be helpful.
Fundamentally, I think you misunderstood the line "sharing stateful logic between components". Stateful logic is different from state. Stateful logic is stuff that you do that modifies state. For e.g., a component subscribing to a store in componentDidMount() and unsubscribing in componentWillUnmount(). This subscribing/unsubscribing behavior can be implemented in a hook and components which need this behavior can just use the hook.
If you want to share state between components, there are various ways to do so, each with its own merits:
1. Lift State Up
Lift state up to a common ancestor component of the two components.
function Ancestor() {
const [count, setCount] = useState(999);
return <>
<DescendantA count={count} onCountChange={setCount} />
<DescendantB count={count} onCountChange={setCount} />
</>;
}
This state sharing approach is not fundamentally different from the traditional way of using state, hooks just give us a different way to declare component state.
2. Context
If the descendants are too deep down in the component hierarchy and you don't want to pass the state down too many layers, you could use the Context API.
There's a useContext hook which you can leverage on within the child components.
3. External State Management Solution
State management libraries like Redux or Mobx. Your state will then live in a store outside of React and components can connect/subscribe to the store to receive updates.

It is possible without any external state management library. Just use a simple observable implementation:
function makeObservable(target) {
let listeners = []; // initial listeners can be passed an an argument aswell
let value = target;
function get() {
return value;
}
function set(newValue) {
if (value === newValue) return;
value = newValue;
listeners.forEach((l) => l(value));
}
function subscribe(listenerFunc) {
listeners.push(listenerFunc);
return () => unsubscribe(listenerFunc); // will be used inside React.useEffect
}
function unsubscribe(listenerFunc) {
listeners = listeners.filter((l) => l !== listenerFunc);
}
return {
get,
set,
subscribe,
};
}
And then create a store and hook it to react by using subscribe in useEffect:
const userStore = makeObservable({ name: "user", count: 0 });
const useUser = () => {
const [user, setUser] = React.useState(userStore.get());
React.useEffect(() => {
return userStore.subscribe(setUser);
}, []);
const actions = React.useMemo(() => {
return {
setName: (name) => userStore.set({ ...user, name }),
incrementCount: () => userStore.set({ ...user, count: user.count + 1 }),
decrementCount: () => userStore.set({ ...user, count: user.count - 1 }),
}
}, [user])
return {
state: user,
actions
}
}
And that should work. No need for React.Context or lifting state up

This is possible using the useBetween hook.
See in codesandbox
import React, { useState } from 'react';
import { useBetween } from 'use-between';
const useShareableState = () => {
const [username, setUsername] = useState('Abrar');
const [count, setCount] = useState(0);
return {
username,
setUsername,
count,
setCount
}
}
const HookComponent = () => {
const { username, setUsername, count } = useBetween(useShareableState);
const handleChange = (e) => {
setUsername(e.target.value);
}
return (
<div>
<input name="userName" value={username} onChange={handleChange}/>
<p>{username}</p>
<p>From HookComponent: {count}</p>
</div>
)
}
const HookComponent2 = () => {
const { count, setCount } = useBetween(useShareableState);
return (
<div>
<p>You clicked {count} times</p>
<button onClick={() => setCount(count + 1)}>
Click me
</button>
</div>
);
}
We move React hooks stateful logic from HookComponent to useShareableState.
We call useShareableState using useBetween in each component.
useBetween is a way to call any hook. But so that the state will not be stored in the React component.
For the same hook, the result of the call will be the same. So we can call one hook in different components and work together on one state. When updating the shared state, each component using it will be updated too.
Disclaimer: I'm the author of the use-between package.

the doc states:
We import the useState Hook from React. It lets us keep local state in a function component.
it is not mentioned that the state could be shared across components, useState hook just give you a quicker way to declare a state field and its correspondent setter in one single instruction.

I've created hooksy that allows you to do exactly this - https://github.com/pie6k/hooksy
import { createStore } from 'hooksy';
interface UserData {
username: string;
}
const defaultUser: UserData = { username: 'Foo' };
export const [useUserStore] = createStore(defaultUser); // we've created store with initial value.
// useUserStore has the same signature like react useState hook, but the state will be shared across all components using it
And later in any component
import React from 'react';
import { useUserStore } from './userStore';
export function UserInfo() {
const [user, setUser] = useUserStore(); // use it the same way like useState, but have state shared across any component using it (eg. if any of them will call setUser - all other components using it will get re-rendered with new state)
function login() {
setUser({ username: 'Foo' })
}
return (
<div>
{!user && <strong>You're logged out<button onPress={login}>Login</button></strong>}
{user && <strong>Logged as <strong>{user.username}</strong></strong>}
</div>
);
}

With hooks its not directly possible.
I recommend you to take a look at react-easy-state.
https://github.com/solkimicreb/react-easy-state
I use it in big Apps and it works like a charm.

I'm going to hell for this:
// src/hooks/useMessagePipe.ts
import { useReducer } from 'react'
let message = undefined
export default function useMessagePipe() {
const triggerRender = useReducer((bool) => !bool, true)[1]
function update(term: string) {
message = term.length > 0 ? term : undefined
triggerRender()
}
return {message: message, sendMessage: update}
}
Full explanation over at: https://stackoverflow.com/a/72917627/1246547
Yes, this is the dirtiest and most concise way i could come up with for solving that specific use case. And yes, for a clean way, you probably want to learn how to useContext, or alternatively take a look at react-easy-state or useBetween for low-footprint solutions, and flux or redux for the real thing.

You will still need to lift your state up to an ancestor component of HookComponent1 and HookComponent2. That's how you share state before and the latest hook api doesnt change anything about it.

Modularizing code in React/Redux

The main question
I am used to using React with ES6 classes. I am also used to modularizing portions of code into separate functions. I am looking at the following example and trying to figure out how to put the value for onSubmit as a separate function.
import React from 'react'
import { connect } from 'react-redux'
import { addTodo } from '../actions'
let AddTodo = ({ dispatch }) => {
let input
return (
<div>
<form
onSubmit={e => {
e.preventDefault()
if (!input.value.trim()) {
return
}
dispatch(addTodo(input.value))
input.value = ''
}}
>
<input
ref={node => {
input = node
}}
/>
<button type="submit">
Add Todo
</button>
</form>
</div>
)
}
AddTodo = connect()(AddTodo)
export default AddTodo
I have tried something like this:
import React from 'react'
import { connect } from 'react-redux'
import { addTodo } from '../actions'
function handleSubmit(e){
e.preventDefault()
if (!input.value.trim()) {
return
}
dispatch(addTodo(input.value))
input.value = ''
}
let AddTodo = ({ dispatch }) => {
let input
return (
<div>
<form onSubmit={e => handleSubmit(e)}>
<input ref={node => {input = node }}
/>
<button type="submit">
Add Todo
</button>
</form>
</div>
)
}
AddTodo = connect()(AddTodo)
export default AddTodo
But then of course it does not work as it does not recognize the input variable. I could pass the input variable to the function, but this does not seem like the right way to do it.
Question 2:
I am unfamiliar with what the following piece of code is doing:
let AddTodo = ({ dispatch }) => {
Where exactly is it getting dispatch from? Is the value of dispatch being passed into the anonymous function?
Question 3
The same with the following code:
<input ref={node => {input = node }}
Where is the value of node coming from and why is it being stored into the input variable?

Answer to Question 1
AddTodo is a React stateless functional component (SFC). It is also a function. Within the SFC is defined a variable input. In order for the handleSubmit callback to be able to make use of input, it is necessary that input be in the enclosing scope where handleSubmit is defined or input be passed as an argument to handleSubmit.
Thus, the following two implementations achieve the desired behavior:
const AddTodo = ({dispatch}) => {
let input
const handleSubmit = e => {
...
}
return (
...
onSubmit={handleSubmit}
...
)
and
const handleSubmit = (e, input) => {
...
}
const AddTodo = ({dispatch}) => {
let input
return (
...
onSubmit={e => handleSubmit(e, input)}
...
)
I highly recommend reading the following blog post by Kent Dodds, paying particular attention to the use of classes vs function closures.
Classes, Complexity, and Functional Programming
Answer to Question 2
The connect function from react-redux wraps the AddTodo component. The way in which connect is being called (with no second argument, or any arguments in this particular case) means AddTodo will receive a prop named dispatch.
To better understand how react-redux and the connect function it provides work, have a look at the documentation:
https://github.com/reactjs/react-redux/blob/master/docs/api.md
Answer to Question 3
Refs are built into React. A function passed to the ref prop receives the underlying DOM element as an argument. In this case, the function passed to the ref prop stores a reference to the DOM element in the variable input. This allows the DOM element to be accessed and mutated later by the callback passed to onSubmit (i.e. handleSubmit). See the React documentation for more details on refs:
https://reactjs.org/docs/refs-and-the-dom.html