function unable to access immediate state change using useEffect - javascript

I am trying to learn how to use Hooks
I am rendering a list of an array remove the first element onclick the state is getting updated in that handleclick function but it not rendering
export default function App() {
const [name, setName] = useState("");
const [arry, setArry] = useState([1, 2, 3, 4, 5]);
const _log = () => {
let ar = arry;
ar.shift();
setArry(ar);
console.log(arry, "log");
};
const renderlist = () => {
console.log(arry, "--");
if (arry.length > 1) {
return arry.map((a) => <li>{a}</li>);
}
}; //
return (
<div className="App">
<button onClick={_log}>shift</button>
{renderlist()}
</div>
);
} ```

React compares objects and arrays by reference instead of by value. This means that what React is checking is the position of memory in which your array is allocated, and not the actual content of your array. As long as that reference remains the same, React won't detect any changes done inside of your array.
let ar = arry;
ar.shift();
setArry(ar);
Your ar variable here is nothing but a pointer to the current arry variable. When you do setArry(ar), what you're doing is assign that same pointer again to your arry value. This is the reason why it's not rendering: since the reference to the array is still the same, React is not aware of the changes you've made.
In order to fix this, you need to create a new array instance with the changes and pass it to setArry.
setArry([...ar]);
Here we're using detructuring to create a copy of your current ar array. Since this copy is a completely different element, it has a different reference. Because of that, calling setArry with this new item will cause your component to re-render with the new values.

You need to pass different value (array) to the setArry. Because useState is optimised to not change value if newValue === currValue.
You pass same array because you mutate it which is anti-pattern (as you see in action) in React.
const _log = () => {
const [_, ...ar] = arry;
setArry(ar);
// or setArry(([_, ...ar]) => ar);
console.log(arry, "log");
};

You need to take a copy of the array before you make a change otherwise you'll still be referencing the original array.
let ar = [...arry];
And change your condition slightly:
if (arry.length > 0) {
// Get a hook function
const {useState} = React;
function Example() {
const [name, setName] = useState("");
const [arry, setArry] = useState([1, 2, 3, 4, 5]);
const _log = () => {
let ar = [...arry];
ar.shift();
setArry(ar);
// console.log(arry, "log");
};
const renderlist = () => {
// console.log(arry, "--");
if (arry.length > 0) {
return arry.map((a) => <li>{a}</li>);
}
}; //
return (
<div className="App">
<button onClick={_log}>shift</button>
{renderlist()}
</div>
);
};
// Render it
ReactDOM.render(
<Example title="Example using Hooks:" />,
document.getElementById("react")
);
<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="react"></div>

Related

force remount on value change

I have some files that builds a cart in a dropdown for my shop website.
One file adds the selected item to an array which will be my cart. The other file is the CartDropdown component itself. My cart only show the items when I close and open it (remounting), but I want it to remount every time I add a new item.
Adding item function:
const ProductContainer = ({ productInfo }) => {
const { cartProducts, setCartProducts } = useContext(CartContext);
const cartArray = cartProducts;
const addProduct = () => {
productInfo.quantity = 1;
if (cartArray.includes(productInfo)) {
const index = cartArray.findIndex((object) => {
return object === productInfo;
});
cartProducts[index].quantity++;
setCartProducts(cartArray);
} else {
cartArray.push(productInfo);
setCartProducts(cartArray);
}
// setCartProducts(cartArray)
console.log(cartProducts);
// console.log(cartArray)
};
};
dropdown component
const CartDropdown = () => {
const { setCartProducts, cartProducts } = useContext(CartContext);
const { setProducts, currentProducts } = useContext(ProductsContext);
// useEffect(() => {}, [cartProducts])
const cleanCart = () => {
const cleanProducts = currentProducts;
console.log(cleanProducts);
for (let i in cleanProducts) {
if (cleanProducts[i].hasOwnProperty("quantity")) {
cleanProducts[i].quantity = 0;
}
}
setProducts(cleanProducts);
setCartProducts([]);
};
return (
<div className="cart-dropdown-container">
<div className="cart-items">
{cartProducts.map((product) => (
<div key={product.id}>
<img src={product.imageUrl}></img>
</div>
))}
</div>
<button onClick={cleanCart}>CLEAN CART</button>
<Button children={"FINALIZE PURCHASE"} />
</div>
);
};
How can I force the dropdown to remount every time cartProducts changes?
CART CONTEXT:
export const CartContext = createContext({
isCartOpen: false,
setIsCartOpen: () => { },
cartProducts: [],
setCartProducts: () => { }
})
export const CartProvider = ({ children }) => {
const [isCartOpen, setIsCartOpen] = useState(false)
const [cartProducts, setCartProducts] = useState([])
const value = { isCartOpen, setIsCartOpen, cartProducts, setCartProducts };
return (
<CartContext.Provider value={value}>{children}</CartContext.Provider>
)
}
product context
export const ProductsContext = createContext({
currentProducts: null,
setProducts: () => {}
})
export const ProductsProvider = ({children}) => {
const [currentProducts, setProducts] = useState(shop_data)
const value = {currentProducts, setProducts}
return(
<ProductsContext.Provider value={value}>{children}</ProductsContext.Provider>
)
}
You can change the key prop of the component every time you want to remount. Every time cartProduct changes, update the value of key. You can do that using a useEffect with cartProduct as a dependency.
<CartDropdown key={1} />
to
<CartDropdown key={2} />
Edit for more clarification:
const [keyCount, setKeyCount] = useState(0);
useEffect(() => {
setKeyCount(keyCount+1);
}, [cartProducts]);
<CartDropdown {...otherProps} key={keyCount} />
The first issue I see is that you are not using the callback to set the state inside the context but you are doing cartProducts[index].quantity++ and react docs specify
Do Not Modify State Directly
Also after cartProducts[index].quantity++, you call setCartProducts(cartArray); not with cartProducts which you actually updated (this is also the reason why "if I do usestate(console.log('A'), [cartProducts]) its not triggering everytime i add my cart product". But anyway there is an issue even if you would use cartArray for both:
You shouldn't directly do const cartArray = cartProducts since by doing so cartArray will be a reference to cartProducts (not a copy of it) which also shouldn't be modified (because it would mean that you are modifying state directly).
So first 2 things I recommend you to improve would be:
Initialize cartArray as a cartProducts deep copy (if your cartProducts is an array of objects, spread syntax won't do it). So I would reccomand you to check this question answers for creating a deep copy.
After you make sure that cartArray is a deep copy of cartProducts, doublecheck you use cartArray to create a local newValue then set the state of the context with the same value (so basically:
cartArray[index].quantity++;
setCartProducts(cartArray);
)
The deep copy part also apply for const cleanProducts = currentProducts; (you should also create a deep copy here for cleanProducts, instead of saving the object ref).
If you are not using deep copies, your code might still work in some cases, but you might encounter weird behaviors in some other instances (and thoose are really hard to debug). Therefore is a bad practice in general not using deep copies.

How to update/add new data to react functional component object?

I am trying to create a function that updates an object in react functional component.
What i was trying to do is:
const [content, setContent] = useState({});
const applyContent = (num: number, key: string, val: string) => {
if (content[num] === undefined) {
content[num] = {};
}
content[num][key] = val;
setNewContent(newInput);
};
But I keep getting an error stating that content doesnt have a num attribute,
In vanilla JS it would work, what am i missing to make it work with react functional component?
The setter for your component state has been incorrectly spelled. Have a look at the code below.
import React, { useState } from 'react';
import './style.css';
export default function App() {
const [content, setContent] = useState({});
const applyContent = (num, key, val) => {
//gets the appropriate inputs
let updatedContent = content;
let value = {};
value[key] = val;
updatedContent[num] = value; //this inserts a new object if not present ot updates the existing one.
setContent({ ...updatedContent });
};
return (
<div>
<h1>Click buttons to change content</h1>
<p>{JSON.stringify(content)}</p>
<button onClick={(e) => applyContent(0, 'a', 'b')}>Add</button>
<button onClick={(e) => applyContent(1, 'c', 'd')}>Add</button>
<button onClick={(e) => applyContent(0, 'e', 'f')}>Add</button>
</div>
);
}
content is a read only value. You must not directly mutate this. Use it only to show data or to copy this data to another helper value.
setContent is a function that sets content.
There are two ways to set data
setContent(value) <-- set directly
setContent(prevState => {
return {
...prevState,
...value
}
})
In second example, you will use the previous value, copy it, and then override it with new value. This is useful if you are only updating a part of an object or array.
If you you are working with a deeply nested object, shallow copy might not be enough and you might need to deepcopy your content value first. If not, then use the prevState example to only update the part of that content
const [content, setContent] = useState({});
const applyContent = (num:number,key:string,val:string) => {
const newContent = {...content} // Shallow copy content
if (content[num] === undefined) {
//content[num] = {}; <-- you cant directly change content. content is a readOnly value
newContent[num] = {}
}
newContent[num][key] = val;
//setNewContent(newInput);
setContent(newContent) // <-- use "setContent" to change "content" value
}

React : useState for arrays with or without arrow function [duplicate]

How to push element inside useState array React hook?
Is that as an old method in react state? Or something new?
E.g. setState push example ?
When you use useState, you can get an update method for the state item:
const [theArray, setTheArray] = useState(initialArray);
then, when you want to add a new element, you use that function and pass in the new array or a function that will create the new array. Normally the latter, since state updates are asynchronous and sometimes batched:
setTheArray(oldArray => [...oldArray, newElement]);
Sometimes you can get away without using that callback form, if you only update the array in handlers for certain specific user events like click (but not like mousemove):
setTheArray([...theArray, newElement]);
The events for which React ensures that rendering is flushed are the "discrete events" listed here.
Live Example (passing a callback into setTheArray):
const {useState, useCallback} = React;
function Example() {
const [theArray, setTheArray] = useState([]);
const addEntryClick = () => {
setTheArray(oldArray => [...oldArray, `Entry ${oldArray.length}`]);
};
return [
<input type="button" onClick={addEntryClick} value="Add" />,
<div>{theArray.map(entry =>
<div>{entry}</div>
)}
</div>
];
}
ReactDOM.render(
<Example />,
document.getElementById("root")
);
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.1/umd/react-dom.production.min.js"></script>
Because the only update to theArray in there is the one in a click event (one of the "discrete" events), I could get away with a direct update in addEntry:
const {useState, useCallback} = React;
function Example() {
const [theArray, setTheArray] = useState([]);
const addEntryClick = () => {
setTheArray([...theArray, `Entry ${theArray.length}`]);
};
return [
<input type="button" onClick={addEntryClick} value="Add" />,
<div>{theArray.map(entry =>
<div>{entry}</div>
)}
</div>
];
}
ReactDOM.render(
<Example />,
document.getElementById("root")
);
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.1/umd/react-dom.production.min.js"></script>
To expand a little further,
here are some common examples. Starting with:
const [theArray, setTheArray] = useState(initialArray);
const [theObject, setTheObject] = useState(initialObject);
Push element at end of array
setTheArray(prevArray => [...prevArray, newValue])
Push/update element at end of object
setTheObject(prevState => ({ ...prevState, currentOrNewKey: newValue}));
Push/update element at end of array of objects
setTheArray(prevState => [...prevState, {currentOrNewKey: newValue}]);
Push element at end of object of arrays
let specificArrayInObject = theObject.array.slice();
specificArrayInObject.push(newValue);
const newObj = { ...theObject, [event.target.name]: specificArrayInObject };
theObject(newObj);
Here are some working examples too.
https://codesandbox.io/s/reacthooks-push-r991u
You can append array of Data at the end of custom state:
const [vehicleData, setVehicleData] = React.useState<any[]>([]);
setVehicleData(old => [...old, ...newArrayData]);
For example, In below, you appear an example of axios:
useEffect(() => {
const fetchData = async () => {
const result = await axios(
{
url: `http://localhost:4000/api/vehicle?page=${page + 1}&pageSize=10`,
method: 'get',
}
);
setVehicleData(old => [...old, ...result.data.data]);
};
fetchData();
}, [page]);
Most recommended method is using wrapper function and spread operator together. For example, if you have initialized a state called name like this,
const [names, setNames] = useState([])
You can push to this array like this,
setNames(names => [...names, newName])
Hope that helps.
// Save search term state to React Hooks with spread operator and wrapper function
// Using .concat(), no wrapper function (not recommended)
setSearches(searches.concat(query))
// Using .concat(), wrapper function (recommended)
setSearches(searches => searches.concat(query))
// Spread operator, no wrapper function (not recommended)
setSearches([...searches, query])
// Spread operator, wrapper function (recommended)
setSearches(searches => [...searches, query])
https://medium.com/javascript-in-plain-english/how-to-add-to-an-array-in-react-state-3d08ddb2e1dc
setTheArray([...theArray, newElement]); is the simplest answer but be careful for the mutation of items in theArray. Use deep cloning of array items.
I tried the above methods for pushing an object into an array of objects in useState but had the following error when using TypeScript:
Type 'TxBacklog[] | undefined' must have a 'Symbol.iterator' method that returns an iterator.ts(2488)
The setup for the tsconfig.json was apparently right:
{
"compilerOptions": {
"target": "es6",
"lib": [
"dom",
"dom.iterable",
"esnext",
"es6",
],
This workaround solved the problem (my sample code):
Interface:
interface TxBacklog {
status: string,
txHash: string,
}
State variable:
const [txBacklog, setTxBacklog] = React.useState<TxBacklog[]>();
Push new object into array:
// Define new object to be added
const newTx = {
txHash: '0x368eb7269eb88ba86..',
status: 'pending'
};
// Push new object into array
(txBacklog)
? setTxBacklog(prevState => [ ...prevState!, newTx ])
: setTxBacklog([newTx]);
if you want to push after specific index you can do as below:
const handleAddAfterIndex = index => {
setTheArray(oldItems => {
const copyItems = [...oldItems];
const finalItems = [];
for (let i = 0; i < copyItems.length; i += 1) {
if (i === index) {
finalItems.push(copyItems[i]);
finalItems.push(newItem);
} else {
finalItems.push(copyItems[i]);
}
}
return finalItems;
});
};

React multiple callbacks not updating the local state

I have a child component called First which is implemented below:
function First(props) {
const handleButtonClick = () => {
props.positiveCallback({key: 'positive', value: 'pos'})
props.negativeCallback({key: 'negative', value: '-100'})
}
return (
<div><button onClick={() => handleButtonClick()}>FIRST</button></div>
)
}
And I have App.js component.
function App() {
const [counter, setCounter] = useState({positive: '+', negative: '-'})
const handleCounterCallback = (obj) => {
console.log(obj)
let newCounter = {...counter}
newCounter[obj.key] = obj.value
setCounter(newCounter)
}
const handleDisplayClick = () => {
console.log(counter)
}
return (
<div className="App">
<First positiveCallback = {handleCounterCallback} negativeCallback = {handleCounterCallback} />
<Second negativeCallback = {handleCounterCallback} />
<button onClick={() => handleDisplayClick()}>Display</button>
</div>
);
}
When handleButtonClick is clicked in First component it triggers multiple callbacks but only the last callback updates the state.
In the example:
props.positiveCallback({key: 'positive', value: 'pos'}) // not updated
props.negativeCallback({key: 'negative', value: '-100'}) // updated
Any ideas?
Both are updating the state, your problem is the last one is overwriting the first when you spread the previous state (which isn't updated by the time your accessing it, so you are spreading the initial state). An easy workaround is to split counter into smaller pieces and update them individually
const [positive, setPositive] = useState('+')
const [negative, setNegative] = useState('-')
//This prevents your current code of breaking when accessing counter[key]
const counter = { positive, negative }
const handleCounterCallback = ({ key, value }) => {
key === 'positive' ? setPositive(value) : setNegative(value)
}
You can do that but useState setter is async like this.setState. If you want to base on the previous value you should use setter as function and you can store it in one state - change handleCounterCallback to
const handleCounterCallback = ({key,value}) => {
setCounter(prev=>({...prev, [key]: value}))
}
and that is all. Always if you want to base on the previous state use setter for the state as function.
I recommend you to use another hook rather than useState which is useReducer - I think it will be better for you

Push method in React Hooks (useState)?

How to push element inside useState array React hook?
Is that as an old method in react state? Or something new?
E.g. setState push example ?
When you use useState, you can get an update method for the state item:
const [theArray, setTheArray] = useState(initialArray);
then, when you want to add a new element, you use that function and pass in the new array or a function that will create the new array. Normally the latter, since state updates are asynchronous and sometimes batched:
setTheArray(oldArray => [...oldArray, newElement]);
Sometimes you can get away without using that callback form, if you only update the array in handlers for certain specific user events like click (but not like mousemove):
setTheArray([...theArray, newElement]);
The events for which React ensures that rendering is flushed are the "discrete events" listed here.
Live Example (passing a callback into setTheArray):
const {useState, useCallback} = React;
function Example() {
const [theArray, setTheArray] = useState([]);
const addEntryClick = () => {
setTheArray(oldArray => [...oldArray, `Entry ${oldArray.length}`]);
};
return [
<input type="button" onClick={addEntryClick} value="Add" />,
<div>{theArray.map(entry =>
<div>{entry}</div>
)}
</div>
];
}
ReactDOM.render(
<Example />,
document.getElementById("root")
);
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.1/umd/react-dom.production.min.js"></script>
Because the only update to theArray in there is the one in a click event (one of the "discrete" events), I could get away with a direct update in addEntry:
const {useState, useCallback} = React;
function Example() {
const [theArray, setTheArray] = useState([]);
const addEntryClick = () => {
setTheArray([...theArray, `Entry ${theArray.length}`]);
};
return [
<input type="button" onClick={addEntryClick} value="Add" />,
<div>{theArray.map(entry =>
<div>{entry}</div>
)}
</div>
];
}
ReactDOM.render(
<Example />,
document.getElementById("root")
);
<div id="root"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.8.1/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.8.1/umd/react-dom.production.min.js"></script>
To expand a little further,
here are some common examples. Starting with:
const [theArray, setTheArray] = useState(initialArray);
const [theObject, setTheObject] = useState(initialObject);
Push element at end of array
setTheArray(prevArray => [...prevArray, newValue])
Push/update element at end of object
setTheObject(prevState => ({ ...prevState, currentOrNewKey: newValue}));
Push/update element at end of array of objects
setTheArray(prevState => [...prevState, {currentOrNewKey: newValue}]);
Push element at end of object of arrays
let specificArrayInObject = theObject.array.slice();
specificArrayInObject.push(newValue);
const newObj = { ...theObject, [event.target.name]: specificArrayInObject };
theObject(newObj);
Here are some working examples too.
https://codesandbox.io/s/reacthooks-push-r991u
You can append array of Data at the end of custom state:
const [vehicleData, setVehicleData] = React.useState<any[]>([]);
setVehicleData(old => [...old, ...newArrayData]);
For example, In below, you appear an example of axios:
useEffect(() => {
const fetchData = async () => {
const result = await axios(
{
url: `http://localhost:4000/api/vehicle?page=${page + 1}&pageSize=10`,
method: 'get',
}
);
setVehicleData(old => [...old, ...result.data.data]);
};
fetchData();
}, [page]);
Most recommended method is using wrapper function and spread operator together. For example, if you have initialized a state called name like this,
const [names, setNames] = useState([])
You can push to this array like this,
setNames(names => [...names, newName])
Hope that helps.
// Save search term state to React Hooks with spread operator and wrapper function
// Using .concat(), no wrapper function (not recommended)
setSearches(searches.concat(query))
// Using .concat(), wrapper function (recommended)
setSearches(searches => searches.concat(query))
// Spread operator, no wrapper function (not recommended)
setSearches([...searches, query])
// Spread operator, wrapper function (recommended)
setSearches(searches => [...searches, query])
https://medium.com/javascript-in-plain-english/how-to-add-to-an-array-in-react-state-3d08ddb2e1dc
setTheArray([...theArray, newElement]); is the simplest answer but be careful for the mutation of items in theArray. Use deep cloning of array items.
I tried the above methods for pushing an object into an array of objects in useState but had the following error when using TypeScript:
Type 'TxBacklog[] | undefined' must have a 'Symbol.iterator' method that returns an iterator.ts(2488)
The setup for the tsconfig.json was apparently right:
{
"compilerOptions": {
"target": "es6",
"lib": [
"dom",
"dom.iterable",
"esnext",
"es6",
],
This workaround solved the problem (my sample code):
Interface:
interface TxBacklog {
status: string,
txHash: string,
}
State variable:
const [txBacklog, setTxBacklog] = React.useState<TxBacklog[]>();
Push new object into array:
// Define new object to be added
const newTx = {
txHash: '0x368eb7269eb88ba86..',
status: 'pending'
};
// Push new object into array
(txBacklog)
? setTxBacklog(prevState => [ ...prevState!, newTx ])
: setTxBacklog([newTx]);
if you want to push after specific index you can do as below:
const handleAddAfterIndex = index => {
setTheArray(oldItems => {
const copyItems = [...oldItems];
const finalItems = [];
for (let i = 0; i < copyItems.length; i += 1) {
if (i === index) {
finalItems.push(copyItems[i]);
finalItems.push(newItem);
} else {
finalItems.push(copyItems[i]);
}
}
return finalItems;
});
};

Categories

Resources