Develop Reference

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.

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;
});
};

function unable to access immediate state change using useEffect

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>

Why the following react functional component does not work properly?

It is quite straight forward, when you press "add" it should add(and it adds) and when you press "remove" it should pop the last element and re-render the list but it doesn't. I am make mistake somewhere?
import React, { useState, useEffect } from 'react';
const Test = () => {
const [list, setList] = useState([]);
const add = () => {
setList([list.length, ...list]);
}
const remove = () => {
list.pop();
setList(list);
}
useEffect(() => {
console.log(list)
}, [list])
return (<ul>
<button onClick={add}>add</button>
<button onClick={remove}>remove</button>
{list.map(el => <li>{el}</li>)}
</ul>)
}
export default Test;
UPDATE:
Actually it updates the state by removing the last element but the re-render happen only when button "add" is pressed

It's not recommended to modify the state itself because it is immutable.
So instead using .pop() on the original state of the array, first I suggest to clone that one and remove the required element from there, then the result should passed to setList() function.
Try as the following instead:
const remove = () => {
const copy = [...list];
copy.pop();
setList(copy);
}
Think about the following:
const list = [1,3,5,6,7];
const copy = [...list];
copy.pop();
console.log(list);
console.log(copy);
I hope this helps!

You need to set a new array in this case, setList(list) will not cause React to re-render because it's still the same array you're using.
Try setList([...list]) in your remove function.
There's also an alternative to pop, and doesn't mutate the original variable:
const remove = () => {
const [removed, ...newList] = list
setList(newList)
}

use object in useEffect 2nd param without having to stringify it to JSON

In JS two objects are not equals.
const a = {}, b = {};
console.log(a === b);
So I can't use an object in useEffect (React hooks) as a second parameter since it will always be considered as false (so it will re-render):
function MyComponent() {
// ...
useEffect(() => {
// do something
}, [myObject]) // <- this is the object that can change.
}
Doing this (code above), results in running effect everytime the component re-render, because object is considered not equal each time.
I can "hack" this by passing the object as a JSON stringified value, but it's a bit dirty IMO:
function MyComponent() {
// ...
useEffect(() => {
// do something
}, [JSON.stringify(myObject)]) // <- yuck
Is there a better way to do this and avoid unwanted calls of the effect?
Side note: the object has nested properties. The effects has to run on every change inside this object.

You could create a custom hook that keeps track of the previous dependency array in a ref and compares the objects with e.g. Lodash isEqual and only runs the provided function if they are not equal.
Example
const { useState, useEffect, useRef } = React;
const { isEqual } = _;
function useDeepEffect(fn, deps) {
const isFirst = useRef(true);
const prevDeps = useRef(deps);
useEffect(() => {
const isFirstEffect = isFirst.current;
const isSame = prevDeps.current.every((obj, index) =>
isEqual(obj, deps[index])
);
isFirst.current = false;
prevDeps.current = deps;
if (isFirstEffect || !isSame) {
return fn();
}
}, deps);
}
function App() {
const [state, setState] = useState({ foo: "foo" });
useEffect(() => {
setTimeout(() => setState({ foo: "foo" }), 1000);
setTimeout(() => setState({ foo: "bar" }), 2000);
}, []);
useDeepEffect(() => {
console.log("State changed!");
}, [state]);
return <div>{JSON.stringify(state)}</div>;
}
ReactDOM.render(<App />, document.getElementById("root"));
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.min.js"></script>
<script src="https://unpkg.com/react#16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom#16/umd/react-dom.development.js"></script>
<div id="root"></div>

The above answer by #Tholle is absolutely correct. I wrote a post regarding the same on dev.to
In React, side effects can be handled in functional components using useEffect hook. In this post, I'm going to talk about the dependency array which holds our props/state and specifically what happens in case there's an object in the dependency array.
The useEffect hook runs even if one element in the dependency array changes. React does this for optimisation purposes. On the other hand, if you pass an empty array then it never re-runs.
However, things become complicated if an object is present in this array. Then even if the object is modified, the hook won't re-run because it doesn't do deep object comparison between these dependency changes for that object. There are couple of ways to solve this problem.
Use lodash's isEqual method and usePrevious hook. This hook internally uses a ref object that holds a mutable current property that can hold values.
It’s possible that in the future React will provide a usePrevious Hook out of the box since it is a relatively common use case.
const prevDeeplyNestedObject = usePrevious(deeplyNestedObject)
useEffect(()=>{
if (
!_.isEqual(
prevDeeplyNestedObject,
deeplyNestedObject,
)
) {
// ...execute your code
}
},[deeplyNestedObject, prevDeeplyNestedObject])
Use useDeepCompareEffect hook as a drop-in replacement for useEffect hook for objects
import useDeepCompareEffect from 'use-deep-compare-effect'
...
useDeepCompareEffect(()=>{
// ...execute your code
}, [deeplyNestedObject])
Use useCustomCompareEffect hook which is similar to solution #2
I prepared a CodeSandbox example related to this post. Fork it and check it yourself.

Your best bet is to use useDeepCompareEffect from react-use. It's a drop-in replacement for useEffect.
const {useDeepCompareEffect} from "react-use";
const App = () => {
useDeepCompareEffect(() => {
// ...
}, [someObject]);
return (<>...</>);
};
export default App;

Plain (not nested) object in dependency array
I just want to challenge these two answers and to ask what happen if object in dependency array is not nested. If that is plain object without properties deeper then one level.
In my opinion in that case, useEffect functionality works without any additional checks.
I just want to write this, to learn and to explain better to myself if I'm wrong. Any suggestions, explanation is very welcome.
Here is maybe easier to check and play with example: https://codesandbox.io/s/usehooks-bt9j5?file=/src/App.js
const {useState, useEffect} = React;
function ChildApp({ person }) {
useEffect(() => {
console.log("useEffect ");
}, [person]);
console.log("Child");
return (
<div>
<hr />
<h2>Inside child</h2>
<div>{person.name}</div>
<div>{person.age}</div>
</div>
);
}
function App() {
const [person, setPerson] = useState({ name: "Bobi", age: 29 });
const [car, setCar] = useState("Volvo");
function handleChange(e) {
const variable = e.target.name;
setPerson({ ...person, [variable]: e.target.value });
}
function handleCarChange(e) {
setCar(e.target.value);
}
return (
<div className="App">
Name:
<input
name="name"
onChange={(e) => handleChange(e)}
value={person.name}
/>
<br />
Age:
<input name="age" onChange={(e) => handleChange(e)} value={person.age} />
<br />
Car: <input name="car" onChange={(e) => handleCarChange(e)} value={car} />
<ChildApp person={person} />
</div>
);
}
ReactDOM.render(<App />, document.getElementById("root"));
<script src="https://unpkg.com/react#16/umd/react.development.js"></script>
<script src="https://unpkg.com/react-dom#16/umd/react-
dom.development.js"></script>
<div id="root"></div>

You can just expand the properties in the useEffect array:
var obj = {a: 1, b: 2};
useEffect(
() => {
//do something when any property inside "a" changes
},
Object.entries(obj).flat()
);
Object.entries(obj) returns an array of pairs ([["a", 1], ["b", 2]]) and .flat() flattens the array into:
["a", 1, "b", 2]
Note that the number of properties in the object must remain constant because the length of the array cannot change or else useEffect will throw an error.