I'm implementing a shopping cart for a ecommerce website. The shopping cart is a state variable shopCart represented by an array of objects. Each object contains information about a product, such as title and price. I am trying to implement a remove button, which is actually doing what is intended from it, which is to remove items from the shopCart state, but the changes are not represented on the screen render. I can empty the cart, but the screen still shows the products. Here is the main code of the shopping cart page:
return (
<div class={styles.container}>
<h1>Product</h1><h1>Quantity</h1><h1>Unit price</h1><h1>Total price</h1><div></div>
{
shopCart.map((product, i, array) => <CartItem key={product.id} product={product} index={i} array={array}/>)
}
</div>
)
And here is the implementation of CartItem.js
const CartItem = (props) => {
let { shopCart, setShopCart } = useContext(Context);
let product = props.product;
// takes the identification of a shopping cart product and removes it from the cart
const decrease = (element) => {
shopCart.forEach((el, i) => {
if (el.hasOwnProperty('id')) {
if (el.id === element) {
let aux = shopCart;
aux.splice(i, 1);
setShopCart(aux);
}
}
})
}
return (
<div>
<img src={product.image}></img>
<h1>{product.quantity}</h1>
<h1>{product.price}</h1>
<h1>{product.price * product.quantity}</h1>
<button onClick={() => {
decrease(product.id);
}}>Remove</button>
</div>
)
}
Why isn't it rendering the cart correctly, even though the cart items are being removed after each click of the remove button ?
Issue
You are mutating state. You save a reference to state, mutate it, then save it back into state, so the array reference never changes. React uses shallow equality when checking if state or props update.
const decrease = (element) => {
shopCart.forEach((el, i) => {
if (el.hasOwnProperty('id')) {
if (el.id === element) {
let aux = shopCart; // <-- Saved state ref
aux.splice(i, 1); // <-- mutation
setShopCart(aux); // <-- Saved ref back to state
}
}
})
}
Solution
The correct way to update arrays in react state is to copy the array elements into a new array reference. This can be easily accomplished by filtering the current cart by item id. I also suggest changing the argument name so it is clearer what it represents.
const decrease = (id) => {
setShopCart(shopCart => shopCart.filter(item => item.id !== id));
}
You're modifying the shopCart (aux is a reference) directly which is both the context and the collection that you're iterating over. You need to make sure you're updating a copy of the shopping cart and resetting the context. Minimally, you can do the following:
const decrease = (element) => {
shopCart.forEach((el, i) => {
if (el.hasOwnProperty('id')) {
if (el.id === element) {
let aux = shopCart.slice(); // makes a copy
aux.splice(i, 1);
setShopCart(aux);
}
}
})
}
However, I suggest using the approach Drew recommended. The current approach isn't ideal.
The solution is much simpler than you think. You can use array.filter to remove the matching product by id.
const CartItem = (props) => {
const { product} = props;
let { shopCart, setShopCart } = useContext(Context);
// takes the identification of a shopping cart product and removes it from the cart
const handleClick = (e) => {
const filteredShopCart = shopCart.filter(item => item.id !== product.id);
setShopCart(filteredShopCart);
};
return (
<div>
<img src={product.image}></img>
<h1>{product.quantity}</h1>
<h1>{product.price}</h1>
<h1>{product.price * product.quantity}</h1>
<button onClick={handleClick}>Remove</button>
</div>
);
};
Related
Solved - wasn't aware of the useRef hook which helped me track each individual mapped item.
I have a set of results mapped out within a card element. I want to keep a click count for each of those elements, but with a global JS variable, it counts the clicks of all elements if I call that variable on more than one clickable element per session. I have tried to do id.index, adding (id) + index etc but am stumped. How do I properly use the unique id's to track the index for each card? Thanks
function onClick(id) {
let index = 0;
index++;
if (index >= 1) {
dosomething
} else if (index === 0) {
dosomethingelse
}
}
It's not clear what and how you want to count and onclick events.
Assuming that you need to keep track of clicks on each element/id:
You can use the useRef hook and keep it a global object to track the number of clicks per id.
const clicksPerId = useRef({});
function onClick(id) {
if (!clicksPerId.current[id]) {
clicksPerId.current[id] = 0;
}
clicksPerId.current[id]++;
// whatever you want to do with the clicks count
}
I'm kinda confused by your question to be honest however for working with arrays in javascript / React maybe you'll find some of these helpful
Getting the array length
const MyComponent = () => {
const [myArray, setMyArray] = useState([1, 2]);
// find the length of the array
const getArrayLength = () => {
return myArray.length;
}
return (
<p>hello there</p>
)
}
Doing something with the index of a maped component:
const MyComponent = () => {
const [myArray, setMyArray] = useState([1, 2]);
const handleClick = (index) => {
// do somthing with the index of the el
};
return (
<>
{ myArray.map((el, index) => {
return (
<p
key={index}
onClick={() => handleClick(index)}
>
el number { el }
</p>
)
})
}
</>
)
}
Is there a way where I can use for loops and if statements without breaking the hook rule? To elaborate, I am currently trying to compare two lists (allData and currentSelection) and if there are similarities, I will add them to another list (favData). However, I am constantly either having visibility issues or errors. If I can get some help, I would much appreciate it!
const [favData, setFavData] = useState([]);
useEffect(() => {
getFilterFavMeal();
}, []);
function getFilterFavMeal() {
allData.forEach((mealList) => {
currentSelection.forEach((mealList2) => {
if (mealList["menu_item"]["menu_item_id"] === mealList2.value) {
// with push, I have visibility issues
// favData.push(mealList);
setFavData(mealList);
}
});
});
setFavData(favData);
}
The set function that useState returns updates the state and schedules a re-render of the component so that the UI can update. It doesn't make sense to call the set function many times in one render.
You also don't want to mutate React state by using functions like push.
Since it looks like favData is deterministic, you can simply remove it from the component state and calculate it in the render loop.
const favData = allData.filter(a => currentSelection.some(c => c.value === a.menu_item.menu_item_id));
Answering your original question, of course you can use loops. As long as you don't mutate the existing state object. And don't set the state more than once per render.
const FF = () => {
const [list, setList] = useState([]);
const addStuffToList = () => {
const tail = Array.from(new Array(3)).map((_e, i) => i);
// Build a new array object and use that when setting state
setList([...list, ...tail]);
}
const forLoop = () => {
const tail = [];
for (let i = 0; i < 4; i++) {
tail.push(i);
}
// Same thing
setList([...list, ...tail]);
}
return ...
};
I have a simple component that allows me to select an item from a list, then remove an item from a list. I display the active list within a parent component. No matter what I do or how I approach it, the removal of an active component is never updated unless they are all in active.
Here is a smaller (yet large snippet) of how it is setup. Below it I describe where I found to be the problem:
const Viewer = () => {
const [items, setItems] = useState(["inactive"]);
return (
<ItemSelect setItems={setItems} selected={items}/>
<DisplayItems items={items}/>
)
}
const ItemSelect = ({setItems, selected}) => {
const handleActiveItems = (activeItems) => {
setItems(activeItems);
}
return (
<SelectItems
handleActiveItems={handleActiveItems}
items={selected}
/>
)
}
const SelectItems = ({handleActiveItems, items}) => {
const [selected, setSelected] = useState([])
useEffect(() => {
setSelected(items);
}, [items]);
const randomTestItem = ["apple", "peach", "orange"];
const handleOnClick = (isSelected, item) => {
let tmpItems = items;
if (isSelected) {
let index = tmpItems.indexOf("inactive");
if (index > -1) {
handleActiveItems([option]);
} else {
handleActiveItems([...selected, option]);
}
} else if (!isSelected) {
let index = tmpItems.indexOf(option);
if (index > -1) {
tmpItems.splice(index, 1);
if (tmpItems.length === 0) {
handleActiveItems(["inactive"]);
} else {
handleActiveItems([tmpItems]);
}
}
}
}
return (
{
randomTestItem?.map((item,index) => {
return (
<DisplayClickable item={item} onClick={handleOnClick} key={index}/>
)
})
}
)
}
<DisplayClickable item={item} onClick={handleOnClick}/> holds a useState() that toggle from active/inactive.
I've tested this in many different area's I believe the crux of the problem to be here:
} else if (!isSelected) {
let index = tmpItems.indexOf(option);
if (index > -1) {
tmpItems.splice(index, 1);
if (tmpItems.length === 0) {
handleActiveItems(["inactive"]);
} else {
handleActiveItems([tmpItems]);
}
}
}
specifically:
} else {
handleActiveItems([tmpItems]);
}
When I unselect all the items and switch the array back to "inactive", everything updates instantly and exactly how you would expect. Selecting items always adds to the list correctly, it's removing them that everything goes wonky. I've done a console.log right before calling handleActiveItems() and the tmpItems array is always correct to what it should be. It just never updates the set state.
Within handleActiveItems the log also shows it is receiving the array just before setting it. It just never sets it.
I believe since you are using the splice method, you just modify the existing array and React does not recognize it as "updatable". You can try to use the filter method:
if (index > -1) {
const newArray = tmpItems.filter((_, itemIndex)=> itemIndex !== index)
if (newArray.length === 0) {
handleActiveItems(["inactive"]);
} else {
handleActiveItems(newArray);
}
}
With the code above, filter method will generate a new array.
Give it a try, hopefully it will help =)
update
I've just realized, maybe you don't need the extra [] you are putting into handleActiveItems(). So instead of:
handleActiveItems([tmpItems])
It could be just:
handleActiveItems(tmpItems)
I figured it out.
It all came down to this line:
let tmpItems = items;
Changing to this:
let tmpItems = [...items];
for some reason allowed React to pay more attention and notice that there was in fact a change.
I just changed in my development build and it works without a hiccup.
There is a local state (hook), it has an array of four elements. There is a button on the screen to add a new element to this array. When a component is loaded, in useEffect called method that removes the first element from the state every 5 seconds. If you do not touch the button that adds a new element to the state, then everything works great. But if you start adding elements, the deletion works according to the previous state, only then the state with the new element is displayed. Tell me how to fix it so that everything works stably. I understand what needs to be sought in the direction of the life cycle, a conflict of states occurs, but I can not find a solution.
const Component = () => {
const [arr, setArr] = useState(['one', 'two', 'three', 'four']);
React.useEffect(() => {
console.log("render");
setTimeout(deleteElementFromArr, 5000)
});
const addNewElementToArr = () => {
let temp = arr.slice();
temp.push('newElement');
setArr(temp);
};
const deleteElementFromArr = () => {
if (arr.length > 0) {
console.log(arr);
let temp = arr.slice();
temp.splice(0, 1);
setArr(temp)
}
};
return (<div>
<div>
<Button onClick={addNewElementToArr}>add</Button>
</div>
<div style={{margiTop: '10px'}}>
{arr.map(a => `${a} `)}
</div>
</div>)
};
https://codepen.io/slava4ka/pen/WNNvrPV
In your useEffect hook, when the effect is finished, clear the timeout. When the state is changed, it will trigger again with the new value of the state.
React.useEffect(() => {
console.log("render");
const timer = setTimeout(deleteElementFromArr, 5000);
return () => clearTimeout(timer);
});
So I got a list of buttons that looks like this
The functionality that I aim for is when you press a button its background will change to another color.
const getUpdatedSelectedItemsArray = (selectedItems, id) => {
selectedItems = []
selectedItems.push(id);
return selectedItems;
};
I use this function to return a list of selected items. Currently I'm only returning one item but I made it an array so I can handle multiple items in the future.
In the render function I have something like this:
<View style={feed_back_page_styles.buttons_wrapper}>
{
feedbackButtons.map((item, i) => (
<TouchableOpacity style={this.state.selectedItems.includes(item.key)?feed_back_page_styles.pressedStyle:feed_back_page_styles.inputStyle}
onPress={()=>this.onButtonPress(item.key)}>
<Text style={this.state.selectedItems.includes(item.key)?feed_back_page_styles.option_text_style_pressed:feed_back_page_styles.option_text_style}>{item.data}</Text>
</TouchableOpacity>
))
}
</View>
feedbackButtons is just an array with a key and text.
The onButtonPress method looks like this:
onButtonPress = (key) =>{
updatedItems = getUpdatedSelectedItemsArray(this.state.selectedItems,key);
this.setState({selectedItems:updatedItems},()=>console.log(this.state.selectedItems));
console.log("Do smth else here");
}
The problem is that the view does not update on state change. When I click the button the state gets updated but the view stays the same.
I think this is wrong
const getUpdatedSelectedItemsArray = (selectedItems, id) => {
selectedItems = []
selectedItems.push(id);
return selectedItems;
};
Since you are passing the this.state.selectedItems as 1st argument from your onButtonPress, actually its not creating new array, but using the same reference of state and state should not be modified directly, always use setState().
So basically what you are doing is :
const getUpdatedSelectedItemsArray = (id) => {
this.state.selectedItems = []
this.state.selectedItems.push(id);
return selectedItems;
};
Which is completely wrong and might be the actual issue.
what you can do instead is :
const getUpdatedSelectedItemsArray = (selectedItems=[], id) => {
const items = [...selectedItems]
items.push(id);
return items;
};
and then :
onButtonPress = (key) =>{
const updatedItems = getUpdatedSelectedItemsArray(key); // since currently you want to keep only 1 item in the list
/* Incase more than 1 items, you can then use this
const updatedItems = getUpdatedSelectedItemsArray(this.state.selectedItems, key);
*/
this.setState({selectedItems:updatedItems},()=>console.log(this.state.selectedItems));
console.log("Do smth else here");
}
Hope this resolves your issue.
Also, if you can share your component, it can help if there is some other issue with your component like if you are using PureComponent.