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Why is the first element getting removed too?

So I am new to javascript and I tried making a todo list. This works well with adding elements. The issue is when I am removing some item, the first one gets removed too, why is it so? I know I am missing a small thing and this may be really basic but I am not able to find out what that is.
const App1 = () => {
const [item, updatedItem]=useState('');
const [Items, setItems]=useState([]);
function inputEvent(event) {
updatedItem(event.target.value);
}
const addItem = (event) => {
event.preventDefault();
setItems((prev) => {
return[
...prev,
item
]
});
updatedItem('');
}
let key=0;
return(<>
<div className='back'>
<div className='list'>
<header>ToDo List</header>
<form onSubmit={addItem}>
<input type='text' placeholder='Add an item' value={item} onChange={inputEvent}/>
<button type='submit'>+</button>
</form>
<div className='items'>
<ol>
{Items.map((val) => <li><button id={key++} onClick={(event) => {
setItems((Items) => {
return Items.filter((val, index) => {
if(index!==Number(event.target.id)){
return index;
}
}
);
});
key=0;
}}>x</button>{val}</li>)}
</ol>
</div>
</div>
</div>
</>);

You return the index in your filter, expecting this to always be true, yet 0 (the index of the first element) is a falsy value.
Try this instead:
return Items.filter((_val, index) => index !== Number(event.target.id));
Some unrelated code-quality notes:
In React, you should always set a key prop on each element when looping through them, rather than id.
map has a second argument, index, which it passes into the callback --- you don't have to keep track of this yourself with e.g. key++ etc.
If you use map's index parameter, then you can pass that directly into your filter rather than using Number(event.target.id), which is not very idiomatic in React.
If you don't use an argument of a callback, it's a good idea to prefix it with a _ (like I've done with _val here), to make it explicit that you're not using it.

Your filter callback should return a flag. index is a number. When treated as a flag, 0 is false (more on MDN). Instead:
return Items.filter((val, index) => index !== Number(event.target.id));
However, your code is returning an array of li elements without setting key on them (see: keys), which React needs in order to manage that list properly (you should be seeing a warning about it in devtools if you're using the development version of the libs, which is best in development). You can't use the mechanism you're using now for keys when doing that, it will not work reliably (see this article linked by the React documentation). Instead, assign each Todo item a unique ID when you create it that doesn't change, and use that as the key (and as the value to look for when removing the item):
// Outside the component:
let lastId = 0;
// Inside the component:
const addItem = (event) => {
event.preventDefault();
setItems((prev) => {
return [
...prev,
{text: item, id: ++lastId}
];
});
updatedItem("");
};
// Add a remove function:
const removeItem = ({currentTarget}) => {
const id = +currentTarget.getAttribute("data-id"); // Get the ID, convert string to number
setItems(items => items.filter(item => item.id !== id));
};
// When rendering:
{Items.map((item) => <li key={item.id}><button data-id={item.id} onClick={removeItem}>x</button>{item.text}</li>)}
In some cases it may be useful to use useCallback to memoize removeItem to avoid unnecessary rendering, but often that's overkill.

ReactJS: Join map output with concatenating value

In my ReactJS application I am getting the mobile numbers as a string which I need to break and generate a link for them to be clickable on the mobile devices. But, instead I am getting [object Object], [object Object] as an output, whereas it should be xxxxx, xxxxx, ....
Also, I need to move this mobileNumbers function to a separate location where it can be accessed via multiple components.
For example: Currently this code is located in the Footer component and this code is also need on the Contact Us component.
...
function isEmpty(value) {
return ((value === undefined) || (value === null))
? ''
: value;
};
function mobileNumbers(value) {
const returning = [];
if(isEmpty(value))
{
var data = value.split(',');
data.map((number, index) => {
var trimed = number.trim();
returning.push(<NavLink to={`tel:${trimed}`} key={index}>{trimed}</NavLink>);
});
return returning.join(', ');
}
return '';
};
...
What am I doing wrong here?
Is there any way to create a separate file for the common constants / functions like this to be accessed when needed?

First question:
What am I doing wrong here?
The issue what you have is happening because of Array.prototype.join(). If creates a string at the end of the day. From the documentation:
The join() method creates and returns a new string by concatenating all of the elements in an array (or an array-like object), separated by commas or a specified separator string. If the array has only one item, then that item will be returned without using the separator.
Think about the following:
const navLinks = [{link:'randomlink'}, {link:'randomlink2'}];
console.log(navLinks.join(','))
If you would like to use concatenate with , then you can do similarly like this:
function mobileNumbers(value) {
if(isEmpty(value)) {
const data = value.split(',');
return data.map((number, index) => {
const trimed = number.trim();
return <NavLink to={`tel:${trimed}`} key={index}>{trimed}</NavLink>;
}).reduce((prev, curr) => [prev, ', ', curr]);
}
return [];
};
Then you need to use map() in JSX to make it work.
Second question:
Is there any way to create a separate file for the common constants / functions like this to be accessed when needed?
Usually what I do for constants is that I create in the src folder a file called Consts.js and put there as the following:
export default {
AppLogo: 'assets/logo_large.jpg',
AppTitle: 'Some app name',
RunFunction: function() { console.log(`I'm running`) }
}
Then simply import in a component when something is needed like:
import Consts from './Consts';
And using in render for example:
return <>
<h1>{Consts.AppTitle}</h1>
</>
Similarly you can call functions as well.
+1 suggestion:
Array.prototype.map() returns an array so you don't need to create one as you did earlier. From the documentation:
The map() method creates a new array populated with the results of calling a provided function on every element in the calling array.
I hope this helps!

Reordering index when mapping arrays with React

I'm changing object property inside mapping and I want to change index when object property is changed ( = input disabled) , so whats best way to do it?
I've tried making new array for index but can't make it work because then it would need some nested mapping with separate arrays and cant make it work.
edit: I use index to mark text part position in official document, thats why this index is so important.
onToggleTextPart = (e, index) => {
const node = this.myCheckbox[index]
let newStates = [ ...this.state.meeting_parts ];
if(node.checked) {
newStates[index - 1].type_tag = "Included";
}
else {
newStates[index - 1].type_tag = "notIncluded";
newStates.splice(index-1, 1)
}
this.setState({ meeting_parts: newStates });
}
return _.map(meeting_parts, (meet, index) => {
let checked = meet.type_tag === "Included" ? true : false;
return
<div className="form-group">
<div className="input-group">
<div className="input-group-prepend">
<span className="input-group-text minutes-agenda-item-number">
{index} (THIS is the index i want to change)
</span>
</div>
I want to i.e when i hide one object from Index 6, it "gives away" its index and Index 7 takes its position.

Okay as far as I understand you want to do this:
meeting_parts.filter((meet)=>meet.type_tag === "Included").map((meet, index)=>{
// your mapping function here
});
Filter will return an array of the meetings which type_tag is "Included".
You can read about the filter function.
EDIT:
let includedCount = 0;
meeting_parts.map((meet, index)=>{
if(meet.type_tag === "Included") {
includedCount += 1;
}
// your mapping function here but use inlcudedCount instead of index
});
Of course now it displays some numbers mutpliple times. If you don't want them displayed you have to add logic to disable the rendering when necessary.

Filter/Reject Array of strings against multiple values using underscore

I'd like to _.filter or _.reject the cities array using the filters array using underscore.
var cities = ['USA/Aberdeen', 'USA/Abilene', 'USA/Akron', 'USA/Albany', 'USA/Albuquerque', 'China/Guangzhou', 'China/Fuzhou', 'China/Beijing', 'China/Baotou', 'China/Hohhot' ... ]
var filters = ['Akron', 'Albuquerque', 'Fuzhou', 'Baotou'];
My progress so far:
var filterList;
if (reject) {
filterList = angular.copy(cities);
_.each(filters, (filter) => {
filterList = _.reject(filterList, (city) => city.indexOf(filter) !== -1);
});
} else {
filterList = [];
_.each(filters, (filter) => {
filterList.push(_.filter(cities, (city) => city.indexOf(filter) !== -1));
});
}
filterList = _.flatten(filterList);
return filterList;
I'd like to DRY this up and use a more functional approach to achieve this if possible?

A somewhat more functional version using Underscore might look like this:
const cities = ['USA/Aberdeen', 'USA/Abilene', 'USA/Akron', 'USA/Albany',
'USA/Albuquerque', 'China/Guangzhou', 'China/Fuzhou',
'China/Beijing', 'China/Baotou', 'China/Hohhot']
const filters = ['Akron', 'Albuquerque', 'Fuzhou', 'Baotou'];
var inList = names => value => _.any(names, name => value.indexOf(name) > -1);
_.filter(cities, inList(filters));
//=> ["USA/Akron", "USA/Albuquerque", "China/Fuzhou", "China/Baotou"]
_.reject(cities, inList(filters));
//=> ["USA/Aberdeen", "USA/Abilene", "USA/Albany",
// "China/Guangzhou", "China/Beijing", "China/Hohhot"]

I'm using vanilla JavaScript here (some() and filter()) but I hope you get the idea:
const isValidCity = city => filters.some(filter => city.indexOf(filter) > -1)
const filteredCities = cities.filter(isValidCity)
Please note that this is a loop over a loop. So the time complexity is O(n * m) here.

In your example all city keys share the same pattern: country + / + city. Your filters are all an exact match to the city part of these names.
If this is a certainty in your data (which it probably isn't...), you could reduce the number of loops your code makes by creating a Map or object that stores each city per filter entry:
Create an object with an entry for each city name
Make the key the part that you want the filter to match
Make the value the original name
Loop through the filters and return the name at each key.
This approach always requires one loop through the data and one loop through the filters. For small array sizes, you won't notice a performance difference. When one of the arrays has length 1, you'll also not notice any differences.
Again, note that this only works if there's a constant relation between your filters and cities.
var cities = ['USA/Aberdeen', 'USA/Abilene', 'USA/Akron', 'USA/Albany', 'USA/Albuquerque', 'China/Guangzhou', 'China/Fuzhou', 'China/Beijing', 'China/Baotou', 'China/Hohhot' ]
var filters = ['Akron', 'Albuquerque', 'Fuzhou', 'Baotou'];
const makeMap = (arr, getKey) => arr.reduce(
(map, x) => Object.assign(map, {
[getKey(x)]: x
}), {}
);
const getProp = obj => k => obj[k];
const getKeys = (obj, keys) => keys.map(getProp(obj));
// Takes the part after the "/"
const cityKey = c => c.match(/\/(.*)/)[1];
const cityMap = makeMap(cities, cityKey);
const results = getKeys(cityMap, filters);
console.log(results);

Since you seem to be using AngularJS, you could utilize the built-in filter functionality. Assuming both the cities and filters array exist on your controller and you're displaying the cities array using ng-repeat, you could have something like this on your controller:
function cityFilter(city) {
var cityName = city.split('/')[1];
if (reject) {
return filters.indexOf(cityName) === -1;
} else {
return filters.indexOf(cityName) > -1;
}
}
And then in your template, you'd do something like this:
<div ng-repeat="city in cities | filter : cityFilter"></div>
Of course you'd have to modify your syntax a bit depending on your code style (for example, whether you use $scope or controllerAs).

Is it possible to map only a portion of an array? (Array.map())

I am building a project using React.js as a front-end framework. On one particular page I am displaying a full data set to the user. I have an Array which contains this full data set. It is an array of JSON objects. In terms of presenting this data to the user, I currently have it displaying the whole data set by returning each item of data using Array.map().
This is a step in the right direction, but now I need to display only a portion of the data-set, not the whole thing, I also want some control in terms of knowing how much of the total data set has been displayed, and how much of the data set is yet to be displayed. Basically I am building something like a "view more" button that loads more items of data to the user.
Here is what I am using now where 'feed' represents my Array of JSON objects. (this displays the whole data set.)
return (
<div className={feedClass}>
{
feed.map((item, index) => {
return <FeedItem key={index} data={item}/>
})
}
</div>
);
I am wondering if it is possible to use .map() on only a portion of the array without having to break up the array before hand? I know that a possible solution would be to hold the full data set, and break it off into portions, and then .map() those portions, but is there a way to .map() a portion of the array without having to break it up?
Any and all feedback is appreciated. Thanks!

Do not try to solve this problem with a hack in your mapping step.
Instead, slice() the list to the right length first before the mapping:
class Feed extends React.Component {
constructor(props) {
super(props)
this.handleShowMore = this.handleShowMore.bind(this)
this.state = {
items: ['Item A', 'Item B', 'Item C', 'Item D'],
showItems: 2
}
}
handleShowMore() {
this.setState({
showItems:
this.state.showItems >= this.state.items.length ?
this.state.showItems : this.state.showItems + 1
})
}
render() {
const items = this.state.items.slice(0, this.state.showItems).map(
(item) => <div>{item}</div>
)
return (
<div>
{items}
<button onClick={this.handleShowMore}>
Show more!
</button>
</div>
)
}
}
ReactDOM.render(
<Feed />,
document.getElementById('root')
)
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>
<div id='root'></div>

The easiest way in my head is just to use a filter and map
const feed = [1,2,3,4,5,6,7,8,9,10,11]
feed.filter((item, index) => index < 5).map((filteredItem) => //do somthing with filtred item here//)
where 5 is just a number of items you want to get

you could use the slice function before to map the array, it looks like you want to do some pagination there.
var fruits = ['Banana', 'Orange', 'Lemon', 'Apple', 'Mango'];
var citrus = fruits.slice(1, 3);
// fruits contains ['Banana', 'Orange', 'Lemon', 'Apple', 'Mango']
// citrus contains ['Orange','Lemon']

Array.reduce should do what you're asking for. Just change the if statement depending on which range you want.
var excludeAfterIndex = 5;
feed.reduce((mappedArray, item, index) => {
if (index > excludeAfterIndex) { // Whatever range condition you want
mappedArray.push(<FeedItem key={index} data={item}/>);
}
return mappedArray;
}, []);

If you just want to map a portion of an array, you should first filter() your array to obtain the expected portion according to conditions :
array.filter(item => <condition>).map();

Yes, you can map portion of array, based on index. For example:
yourArray = yourArray.map(function (element, index, array) {
if (array.indexOf(element) < yourIndex) {
return {
//logic here
};
} else {
return {
//logic here
};
}
});

You can use slice to get portion of an array:
const data = [1,2,3,4,5,6,7,8,9,10,11]
var updatedData = data.slice(0, 3);

Array#map iterates over all items.
The map() method creates a new array with the results of calling a provided function on every element in this array.
You could use Array#filter
The filter() method creates a new array with all elements that pass the test implemented by the provided function.
for the wanted items and then apply map for the wanted format.

There is no version of the map() function that only maps a partial of the array.
You could use .map() in conjunction with .filter().
You get the index of the current element as the second arg of map and if you have a variable for current page and page size you can quite easily filter the right page from your array without having to really slice it up.
var currentPage = 1;
var pageSize = 25;
dataArray.filter(function(elt, index) {
var upperThreshold = currentPage * pageSize;
var lowerThreshold = currentPage * pageSize - pageSize;
return index < upperThreshold && index > lowerThreshold;
});

Using slice() is better than adding a condition to your map or reduce function, but it still creates an additional, unused copy of that segment of the array. Depending on what you're doing, that might not be desired. Instead, just use a custom map function:
function sliceMap(fn, from, toExclusive, array) {
const len = toExclusive - from;
const mapped = Array(len);
for (let i = 0; i < len; i++) {
mapped[i] = fn(array[i + from], i);
}
return mapped;
};
Note that fn receives the array value and the (now) zero-based index. You might want to pass the original index (i + from). You might also want to pass the full array as a third parameter, which is what Array.map does.

Use this, easy approach
const [limit, setLimit] = useState(false);
const data = [{name: "john}, {name: 'Anna'}]
Here we will have 2 cases:
Display only first data which is John
Display all
data.slice(0, extended ? data.length : 1).map((item, index) => <Text>{item.name}</Text>)
....