I am trying to use the state hook in my react app.
But setTodos below seems not updating the todos
link to my work: https://kutt.it/oE2jPJ
link to github: https://github.com/who-know-cg/Todo-react
import React, { useState } from "react";
import Main from "./component/Main";
const Application = () => {
const [todos, setTodos] = useState([]);
// add todo to state(todos)
const addTodos = message => {
const newTodos = todos.concat(message);
setTodos(newTodos);
};
return (
<>
<Main
addTodos={message => addTodos(message)}
/>
</>
);
};
export default Application;
And in my main.js
const Main = props => {
const input = createRef();
return (
<>
<input type="text" ref={input} />
<button
onClick={() => {
props.addTodo(input.current.value);
input.current.value = "";
}}
>
Add message to state
</button>
</>
);
};
I expect that every time I press the button, The setTodos() and getTodos() will be executed, and the message will be added to the todos array.
But it turns out the state is not changed. (still, stay in the default blank array)
If you want to update state of the parent component, you should pass down the function from the parent to child component.
Here is very simple example, how to update state with hook from child (Main) component.
With the help of a button from child component you update state of the parent (Application) component.
const Application = () => {
const [todos, setTodos] = useState([]);
const addTodo = message => {
let todosUpdated = [...todos, message];
setTodos(todosUpdated);
};
return (
<>
<Main addTodo={addTodo} />
<pre>{JSON.stringify(todos, null, 2)}</pre>
</>
);
};
const Main = props => {
const input = createRef();
return (
<>
<input type="text" ref={input} />
<button
onClick={() => {
props.addTodo(input.current.value);
input.current.value = "";
}}
>
Add message to state
</button>
</>
);
};
Demo is here: https://codesandbox.io/s/silent-cache-9y7dl
In Application.jsx :
You can pass just a reference to addTodos here. The name on the left can be whatever you want.
<Main addTodos={addTodos} />
In Main.jsx :
Since getTodo returns a Promise, whatever that promise resolves to will be your expected message.
You don't need to pass message as a parameter in Main, just the name of the function.
<Main addTodos={addTodos} />
You are passing addTodos as prop.
<Main
addTodos={message => addTodos(message)}
/>
However, in child component, you are accessing using
props.addTodo(input.current.value);
It should be addTodos.
props.addTodos(input.current.value);
Related
**I'm getting react Error while trying to change parent component styles onMouseEnter event. How could I change the styles via child component button?
The error is - Uncaught Error: Too many re-renders. React limits the number of renders to prevent an infinite loop.
That seems odd for onMouseLeave={() => setHovered(false)} is an arrow function.
https://codesandbox.io/s/trusting-moon-djocul?
**
// App js
import React, { useState, useEffect } from "react";
import Categories from "./Categories";
import ShopPage from "./components/Products";
export default function App() {
const [data, setData] = useState(Categories);
useEffect(() => {
setData(data);
}, []);
return (
<div className="wrapper">
<ShopPage products={data} filterResult={filterResult} />
</div>
);
}
// Shopping page
const ShopPage = ({ products }) => {
return (
<>
<div>
<Products products={products} />
</div>
</>
);
};
// Parent component, the main part goes here
const Products = ({products}) => {
const [hovered, setHovered] = useState(false);
const [style, setStyle] = useState(false);
if (hovered) {
setStyle({
// inline styles
});
} else {
setStyle({
// inline styles
});
}
return (
<>
<Product setHovered={setHovered} style={style} products={products}/>
</>
);
};
export default Products;
// Child component
const Product = ({ setHovered, style, products }) => {
return (
<div className={styles.items}>
{products.map((value) => {
return (
<>
<div style={style}>
<button
onMouseEnter={() => setHovered(true)}
onMouseLeave={() => setHovered(false)}
>
Add to Cart
</button>
</div>
</>
);
})}
</div>
);
};
export default Product;
The issue is you are setting setHovered state in component, the simple solution could be to use it in useEffect and add required dependency.
If we talk about your code so you can easily do this by using the state in child component instead of passing through props.
I have updated your code below:
https://codesandbox.io/s/nameless-cookies-e6pwxn?file=/src/components/Product.js:141-151
Within a child component I have state to store input.
My goal is to pass the input state to the parent component.
//Child component
const [userInput, setUserInput] = useState("");
<TextField value={input} onInput={e => setInput(e.target.value)} />
I tried creating a function to be called on input within the parent function but I cannot seem to get the value "input" passed. What is the correct way to do this? Thanks!
you were approaching correctly. You can pass a function to get the data from the child to the parent. Check this example:
import { useState } from "react";
import "./styles.css";
export default function App() {
const [text, setText] = useState("");
const getInput = (t) => {
console.log(t);
setText(t);
};
return (
<div className="App">
<Component getInput={getInput} value={text} />
<p>{text}</p>
</div>
);
}
const Component = ({ getInput, value }) => {
return (
<>
<h1> Test </h1>
<input type="text" value={value} onChange={(e) => getInput(e.target.value)}></input>
</>
);
};
The parent App is retrieving the text from the child Component passing the function getInput. You only have to decide with event you want to detonate. Of course, this is with input not with TextField. I think that you are using TextField from React Material UI, if that's the case you may check the docs to know how to trigger an event.
You can move the state to the parent component. Then pass the props from the child to the parent.
function Child({
...rest
}) {
return <TextField {...rest} />
}
function Parent() {
const [input, setInput] = useState(''); // '' is the initial state value
return (
<Child value={value} onInput={e => setInput(e.target.value)} />
)
}
I finally figured this out.
// This is in the parent component
const [userInput, setUserInput] = useState("");
// Step 1: Create a function that handles setting the state
const inputHandler = (userInput) => {
setUserInput(userInput);
}
<PhaseTwoTemplate onInput={inputHandler} />
//Step 2: Add a function that calls the handler
...
// This is the child component PhaseTwoTemplete.js
const [input, setInput] = useState("");
// This second state holds the input data for use on child component
// It also is an easy way to hold the data for the parent function.
<TextField
onChange={e => setInput(e.target.value)}
value={input}
onInput={onInputHandler}
/>
// Step 3: Pass the user input to the parent
const onInputHandler = () => {
props.onInput(input);
}
I have a variable that I don't want to bind to state in React. So I declared it as let with initial value as null. Later with event I set its value in parent and then pass it to child. But in child its value is getting null. Not sure what mistake I am making. Below is the code.
function Parent() {
const[showChild, setShowChild] = useState(false);
let data = null;
const setData = () => {
data = 'Test';
setShowChild(true);
console.log('function called');
};
return (
<>
<button onClick={setData}>
Click Me
</button>
{showChild && <Child data={data} />}
</>
);
}
function Child({data}) {
console.log('data ' + data);
return (
<>
<h2>
{data}
</h2>
</>
);
}
ReactDOM.render(
<Parent />,
document.getElementById('mountNode'),
);
If you want to persist state you have to use useState as you did in the first line. so instead of manually let data ... and const setData you should have something like the following:
const [data, setData] = useState(null)
You sould use useState, whenever the state changes in parent, child will be render again.
I changed your example a little bit to show you what happen exactly, child just shows the value that parent has sent.
const {useState} = React;
const Parent=()=> {
const[showChild, setShowChild] = useState(false);
const[data, setData] = useState(0);
//let data = null;
const handleClick = () => {
setData((prev=>prev+1));
setShowChild(true);
console.log('function called');
};
return (
<div>
<button onClick={handleClick}>
Click Me
</button>
{showChild && <Child data={data} />}
</div>
);
}
const Child=({data})=> {
console.log('data ' + data);
return (
<div>
<h2>
{data}
</h2>
</div>
);
}
ReactDOM.render( <Parent/> ,
document.getElementById("mountNode")
);
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.1/umd/react-dom.production.min.js"></script>
<div id="mountNode"></div>
I need to show the props value (which is a simple string). Each time I get new search results, I'm sending in the props. At the very first render the props will always be undefined.
Edit:
Header.jsx
function Header() {
const [searchString, setString] = useState('');
const onChangHandler = (e) => {
setString(e.target.value);
};
const activeSearch = () => {
if (searchString.length > 0) {
<Home searchResults={searchString} />;
}
};
return (
<div>
<input
placeholder='Search here'
value={searchString}
onChange={(e) => onChangHandler(e)}
/>
<button onClick={activeSearch}>Search</button>
</header>
</div>
);
}
I searched for previous stackoverflow questions and reactjs.org but found no answer.
Home.jsx
import React, { useEffect, useState } from 'react';
function Home({ searchResults }) {
const [itemSearchResults, setResults] = useState([]);
const [previousValue, setPreviousValue] = useState();
// What function will re-render when the props are first defined or changed ?
useEffect(() => { // Doesn't work
setResults(searchResults);
}, [searchResults]);
return (
<div>
<h3>Home</h3>
<h1>{itemSearchResults}</h1>
</div>
);
}
export default Home;
App.js
function App() {
return (
<div className='App'>
<Header />
<Home />
<Footer />
</div>
);
}
I'm sending the input string only to check if the props will change at the child component ("Home").
Any experts here know what's the problem?
Why it doesn't work?
It's because the Home component is never used, even if it's included in the following snippet:
const activeSearch = () => {
if (searchString.length > 0) {
<Home searchResults={searchString} />;
}
};
The activeSearch function has a couple problems:
it is used as an event handler though it uses JSX (outside the render phase)
it doesn't return the JSX (would still fail inside the render phase)
JSX should only be used within the render phase of React's lifecycle. Any event handler exists outside this phase, so any JSX it might use won't end up in the final tree.
The data dictates what to render
That said, the solution is to use the state in order to know what to render during the render phase.
function Header() {
const [searchString, setString] = useState('');
const [showResults, setShowResults] = useState(false);
const onChangHandler = (e) => {
// to avoid fetching results for every character change.
setShowResults(false);
setString(e.target.value);
};
const activeSearch = () => setShowResults(searchString.length > 0);
return (
<div>
<input
value={searchString}
onChange={(e) => onChangHandler(e)}
/>
<button onClick={activeSearch}>Search</button>
{showResults && <Home query={searchString} />}
</div>
);
}
useEffect to trigger effects based on changing props
And then, the Home component can trigger a new search request to some service through useEffect.
function Home({ query }) {
const [results, setResults] = useState(null);
useEffect(() => {
let discardResult = false;
fetchResults(query).then((response) => !discardResult && setResults(response));
// This returned function will run before the query changes and on unmount.
return () => {
// Prevents a race-condition where the results from a previous slow
// request could override the loading state or the latest results from
// a faster request.
discardResult = true;
// Reset the results state whenever the query changes.
setResults(null);
}
}, [query]);
return results ? (
<ul>{results.map((result) => <li>{result}</li>))}</ul>
) : `Loading...`;
}
It's true that it's not optimal to sync some state with props through useEffect like the article highlights:
useEffect(() => {
setInternalState(externalState);
}, [externalState]);
...but in our case, we're not syncing state, we're literally triggering an effect (fetching results), the very reason why useEffect even exists.
const { useState, useEffect } = React;
const FAKE_DELAY = 5; // seconds
function Home({ query }) {
const [results, setResults] = useState(null);
useEffect(() => {
let queryChanged = false;
console.log('Fetch search results for', query);
setTimeout(() => {
if (queryChanged) {
console.log('Query changed since last fetch, results discarded for', query);
return;
}
setResults(['example', 'result', 'for', query])
}, FAKE_DELAY * 1000);
return () => {
// Prevent race-condition
queryChanged = true;
setResults(null);
};
}, [query]);
return (
<div>
{results ? (
<ul>
{results.map((result) => (
<li>{result}</li>
))}
</ul>
) : `Loading... (${FAKE_DELAY} seconds)`}
</div>
);
}
function Header() {
const [searchString, setString] = useState('');
const [showResults, setShowResults] = useState(false);
const onChangHandler = (e) => {
// to avoid fetching results for every character change.
setShowResults(false);
setString(e.target.value);
};
const activeSearch = () => setShowResults(searchString.length > 0);
return (
<div>
<input
placeholder='Search here'
value={searchString}
onChange={(e) => onChangHandler(e)}
/>
<button onClick={activeSearch}>Search</button>
{showResults && <Home query={searchString} />}
</div>
);
}
ReactDOM.render(<Header />, document.querySelector("#app"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.1/umd/react-dom.production.min.js"></script>
<div id="app"></div>
Better solution: Uncontrolled inputs
Another technique in your case would be to use an uncontrolled <input> by using a ref and only updating the search string on click of the button instead of on change of the input value.
function Header() {
const [searchString, setString] = useState('');
const inputRef = useRef();
const activeSearch = () => {
setString(inputRef.current.value);
}
return (
<div>
<input ref={inputRef} />
<button onClick={activeSearch}>Search</button>
{searchString.length > 0 && <Home query={searchString} />}
</div>
);
}
const { useState, useEffect, useRef } = React;
const FAKE_DELAY = 5; // seconds
function Home({ query }) {
const [results, setResults] = useState(null);
useEffect(() => {
let queryChanged = false;
console.log('Fetch search results for', query);
setTimeout(() => {
if (queryChanged) {
console.log('Query changed since last fetch, results discarded for', query);
return;
}
setResults(['example', 'result', 'for', query])
}, FAKE_DELAY * 1000);
return () => {
// Prevent race-condition
queryChanged = true;
setResults(null);
};
}, [query]);
return (
<div>
{results ? (
<ul>
{results.map((result) => (
<li>{result}</li>
))}
</ul>
) : `Loading... (${FAKE_DELAY} seconds)`}
</div>
);
}
function Header() {
const [searchString, setString] = useState('');
const inputRef = useRef();
const activeSearch = () => {
setString(inputRef.current.value);
}
return (
<div>
<input
placeholder='Search here'
ref={inputRef}
/>
<button onClick={activeSearch}>Search</button>
{searchString.length > 0 && <Home query={searchString} />}
</div>
);
}
ReactDOM.render(<Header />, document.querySelector("#app"))
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.1/umd/react-dom.production.min.js"></script>
<div id="app"></div>
Passing the state around
[The following line] brings the Home component inside the Header component, which makes duplicate
{searchString.length > 0 && <Home query={searchString} />}
In order to make the Header component reusable, the quickest way would be to lift the state up.
// No state needed in this component, we now receive
// a callback function instead.
function Header({ onSubmit }) {
const inputRef = useRef();
const activeSearch = () => {
// Uses the callback function instead of a state setter.
onSubmit(inputRef.current.value);
}
return (
<div>
<input ref={inputRef} />
<button onClick={activeSearch}>Search</button>
</div>
);
}
function App() {
// State lifted up to the parent (App) component.
const [searchString, setString] = useState('');
return (
<div className='App'>
<Header onSubmit={setString} />
{searchString.length > 0 && <Home query={searchString} />}
<Footer />
</div>
);
}
If that solution is still too limited, there are other ways to pass data around which would be off-topic to bring them all up in this answer, so I'll link some more information instead:
Thinking in React
What's the right way to pass form element state to sibling/parent elements?
Passing data to sibling components with react hooks?
Application State Management with React
How can I update the parent's state in React?
Top 5 React state management libraries in late 2020 (Redux, Mobx, Recoil, Akita, Hookstate)
if your props are passed as searchResults, then change the props to,
function Home({ searchResults}) {...}
and use
useEffect(() => { // code, function },[searchResults]) ).
I have this handleSubmit that returns me a key (verifyCode) that I should use in another component. How can I pass this verifyCode to another component?
const SendForm = ({ someValues }) => {
const handleSubmitAccount = () => {
dispatch(createAccount(id, username))
.then((response) => {
// I get this value from data.response, its works
const { verifyCode } = response;
})
.catch(() => {
});
};
return(
//the form with handleSubmitAccount()
)
}
export default SendForm;
The other component is not a child component, it is loaded after this submit step. But I don't know how to transfer the const verifyCode.
This is the view where the components are loaded, it's a step view, one is loaded after the other, I need to get the const verifyCode in FormConfirmation
<SendForm onSubmit={handleStepSubmit} onFieldSubmit={handleFieldSubmit} />
<FormConfirmation onSubmit={handleStepSubmit} onFieldSubmit={handleFieldSubmit} />
Does anyone know how I can do this?
You need to move up the state to a component that has both as children and then pass down a function that updates as a prop
import React from "react";
export default function App() {
const [value, setValue] = React.useState(0);
return (
<div className="App">
<Updater onClick={() => setValue(value + 1)} />
<ValueDisplay number={value} />
</div>
);
}
const Updater = (props) => <div onClick={props.onClick}>Update State</div>;
const ValueDisplay = (props) => <div>{props.number}</div>;
Check out the docs here
For more complex component structures or where your passing down many levels you may want to look into reactContext
import React from "react";
//Set Default Context
const valueContext = React.createContext({ value: 0, setValue: undefined });
export default function App() {
const [value, setValue] = React.useState(0);
return (
<div className="App">
{/** Pass in state and setter as value */}
<valueContext.Provider value={{ value: value, setValue }}>
<Updater />
<ValueDisplay />
</valueContext.Provider>
</div>
);
}
const Updater = () => {
/** Access context with hook */
const context = React.useContext(valueContext);
return (
<div onClick={() => context.setValue(context.value + 1)}>Update State</div>
);
};
const ValueDisplay = () => {
/** Access context with hook */
const context = React.useContext(valueContext);
return <div>{context?.value}</div>;
};