I am trying to setState of a component from an array of values.
I have these values on filtersSelected array->
["name", "date", "state"]
I want to set these values to my state like this
myState = {
...etc,
name: null,
date: null,
state: null
}
I tried using
this.setState(previousState => ({
...previousState,
...filtersSelected: null
}))
apparently it doesn't work.Can anyone help me?
In order to spread the array into an object you first need to convert the array into an object and then you can spread the object keys into the state:
this.setState((prevState) => ({
...prevState,
...filtersSelected.reduce(function(acc, item) {
return Object.assign(acc, {[item]: null})
}, {});
}))
There are a couple things to note here. First of all, when you call setState, you do not need to provide all of the previous state. setState "merges" the specific state properties that you pass into the already existing state.
It also might be helpful to make your filters selected array an object, as you are trying to use the spread , but is by no means necessary. If you want to keep the array, you can use the code below.
let filtersSelected = ["name", "date", "state"];
this.setState({name: filtersSelected[0], date: filtersSelected[1], state: filtersSelected[2]});
Or, if you make filtersSelected into an object (which I highly recommend), you can do:
let filtersSelected = {name: "name", date: "date", state: "state"};
this.setState(filtersSelected);
Convert your array to object first then use a loop to assign values to your newly created object.
let filteredSelected = ["name", "date", "state"];
let obj;
for(let i of filteredSelected){
obj[i] = null;
}
this.setState(previousState => ({
...previousState,
obj
}))
Everyone has his own way, here are some ways I like
let data = ["name", "date", "state"];
Object.values(data).map(i => this.setState({ [i]: null }))
But I don't like to iterate setState for each element
let data = Object.assign({}, ...["name", "date", "state"].map((prop) => ({[prop]: null})))
this.setState(data)
Or you can do like so
this.setState(["name", "date", "state"].reduce( ( accumulator, currentValue ) => ({...accumulator,[ currentValue ]: null}), {} ));
Related
I seem to be having issues pushing data into a state array.
I am trying to achieve it this way:
this.setState({ myArray: this.state.myArray.push('new value') })
But I believe this is incorrect way and causes issues with mutability?
Using es6 it can be done like this:
this.setState({ myArray: [...this.state.myArray, 'new value'] }) //simple value
this.setState({ myArray: [...this.state.myArray, ...[1,2,3] ] }) //another array
Spread syntax
Array push returns length
this.state.myArray.push('new value') returns the length of the extended array, instead of the array itself.Array.prototype.push().
I guess you expect the returned value to be the array.
Immutability
It seems it's rather the behaviour of React:
NEVER mutate this.state directly, as calling setState() afterwards may
replace the mutation you made. Treat this.state as if it were
immutable.React.Component.
I guess, you would do it like this (not familiar with React):
var joined = this.state.myArray.concat('new value');
this.setState({ myArray: joined })
Functional Components & React Hooks
const [array,setArray] = useState([]);
Push value at the end:
setArray(oldArray => [...oldArray,newValue] );
Push value at the start:
setArray(oldArray => [newValue,...oldArray] );
Never recommended to mutate the state directly.
The recommended approach in later React versions is to use an updater function when modifying states to prevent race conditions:
Push string to end of the array
this.setState(prevState => ({
myArray: [...prevState.myArray, "new value"]
}))
Push string to beginning of the array
this.setState(prevState => ({
myArray: ["new value", ...prevState.myArray]
}))
Push object to end of the array
this.setState(prevState => ({
myArray: [...prevState.myArray, {"name": "object"}]
}))
Push object to beginning of the array
this.setState(prevState => ({
myArray: [ {"name": "object"}, ...prevState.myArray]
}))
You should not be operating the state at all. At least, not directly. If you want to update your array, you'll want to do something like this.
var newStateArray = this.state.myArray.slice();
newStateArray.push('new value');
this.setState(myArray: newStateArray);
Working on the state object directly is not desirable. You can also take a look at React's immutability helpers.
https://facebook.github.io/react/docs/update.html
Here you can not push the object to a state array like this. You can push like your way in normal array.
Here you have to set the state,
this.setState({
myArray: [...this.state.myArray, 'new value']
})
You can use .concat method to create copy of your array with new data:
this.setState({ myArray: this.state.myArray.concat('new value') })
But beware of special behaviour of .concat method when passing arrays - [1, 2].concat(['foo', 3], 'bar') will result in [1, 2, 'foo', 3, 'bar'].
Using react hooks, you can do following way
const [countryList, setCountries] = useState([]);
setCountries((countryList) => [
...countryList,
"India",
]);
This Code work for me :
fetch('http://localhost:8080')
.then(response => response.json())
.then(json => {
this.setState({mystate: this.state.mystate.push.apply(this.state.mystate, json)})
})
React-Native
if u also want ur UI (ie. ur flatList) to be up to date, use PrevState:
in the example below if user clicks on the button , it is going to add a new object to the list( both in the model and UI)
data: ['shopping','reading'] // declared in constructor
onPress={() => {this.setState((prevState, props) => {
return {data: [new obj].concat(prevState.data) };
})}}.
In the following way we can check and update the objects
this.setState(prevState => ({
Chart: this.state.Chart.length !== 0 ? [...prevState.Chart,data[data.length - 1]] : data
}));
setState([...prevState, {
label: newState.name,
value: newState.id
}]);
Was working with the dropdowns and wanted to implement this scenario there, i found this simple solution for dropdown with multiple values.
you are breaking React principles, you should clone the old state then merge it with the new data, you shouldn't manipulate your state directly,
your code should go like this
fetch('http://localhost:8080').then(response => response.json()).then(json ={this.setState({mystate[...this.state.mystate, json]}) })
If you use:
const[myArr, setMyArr] = useState([]);
for add:
setMyArr([...myArr, value]);
and for remove:
let index = myArr.indexOf(value);
if(index !== -1)
setPatch([...myArr.slice(0, index), ...myArr.slice(index, myArr.length-1)]);
I guess this is a little bit late for an answer but for those new to react
You can use this tiny package called immer
see this example: https://immerjs.github.io/immer/produce
I have an array of objects. In that array, I want to update a single object, and in that object, I want to update only specific properties. I tried:
setRequiredFields(prevRequiredFields => ([
...prevRequiredFields, prevRequiredFields.find(x => x.name = field) ? {
isValid: isValid,
content: content,
isUnchanged: false
}
]));
But it didn't work. Required fileds is an array with the following structure:
[{
name: "Name",
isValid: false,
content: "",
isUnchanged: true,
tip: "Name cannot be empty",
isValidCondition: notEmptyRegex,
reportState: validateField
}]
What I'm trying to do here is to update only a isValid, content and isUnchanged of one specific object inside that array. How can I accomplish that?
If you have an array of objects, and you want to update few properties of one of the objects inside the array. Then you could do something like this.
const index = state.findIndex(x => x.name === field);
if(index > -1) {
const newState = [...state];
newState[index] = {
...newState[index],
isValid: isValid,
content: content,
isUnchanged: false
}
setRequiredFields(newState);
}
Find the index of the object that you want to update.
Add properties inside that object.
Update the react state.
I am receiving a json response from an API call. I need to store its keys, and create an array of an object. I am intending to this array of an object is created dynamically no matter the keys of the response.
I've already got the keys like this:
const json_getAllKeys = data => {
const keys = data.reduce((keys, obj) => (
keys.concat(Object.keys(obj).filter(key => (
keys.indexOf(key) === -1))
)
), [])
return keys
}
That returned an array (using a sample json):
['name','username', 'email']
But I am trying to use that array to create an array of object that looks like this one
[
{
name: "name",
username: "username",
email: "Email",
}
];
I've been trying mapping the array, but got multiple objects because of the loop, and I need a single one to make it work.
keys.map(i=>({i:i}))
[
{ i: 'id' },
{ i: 'name' },
{ i: 'username' },
{ i: 'email' }
]
Any hint would be useful!
Thanks in advance :D
What you're looking for is Object.fromEntries, which is ECMA2019, I believe, so available in Node >=14 and will be provided as a polyfill if you employ babel.
I can't quite discern what your reduce should produce, but given the sample input, I would write
const input = ['name','username', 'email'];
const result = Object.fromEntries(input.map(name => ([name, name])));
// result == { name: 'name', username: 'username', email: 'email' }
You're definitely on the right track. One thing to remember is the map function will return the SAME number of output as input. So in your example, an array of 3 returns an array of 3 items.
For this reason, map alone is not going to give you what you want. You may be able to map => reduce it. However, here is a way using forEach instead. This isn't a strictly functional programming style solution, but is pretty straight forward and depending on use case, probably good enough.
let keys = ['name','username', 'email'] //you have this array
const obj = {}; // empty object to hold result
keys.forEach(i => {
obj[i] = i; // set the object as you want
})
console.log(obj); // log out the mutated object
// { name: 'name', username: 'username', email: 'email' }
I'm trying to come up with the .map call that would use a certain field of a dict as key of the result:
input=[ {key:"name", value:"John"}, {key:"city", value:"Chicago"}]
output = input.map( e => **magic here** );
>> output = [ {name:"John"}, {city:"Chicago"}]
I've tried from input.map( e => { e.name:e.value}), to no avail.
Also tried input.map(({key, value}) => ({key:value})), but it does not populate the key value correctly - rather taking "key" as the key of the dicts.
What am I doing wrong here?
Thanks!
You could destructure the object and take computed property name for a new object.
const
input = [{ key: "name", value: "John" }, { key: "city", value: "Chicago" }],
output = input.map(({ key, value }) => ({ [key]: value }));
console.log(output);
I seem to be having issues pushing data into a state array.
I am trying to achieve it this way:
this.setState({ myArray: this.state.myArray.push('new value') })
But I believe this is incorrect way and causes issues with mutability?
Using es6 it can be done like this:
this.setState({ myArray: [...this.state.myArray, 'new value'] }) //simple value
this.setState({ myArray: [...this.state.myArray, ...[1,2,3] ] }) //another array
Spread syntax
Array push returns length
this.state.myArray.push('new value') returns the length of the extended array, instead of the array itself.Array.prototype.push().
I guess you expect the returned value to be the array.
Immutability
It seems it's rather the behaviour of React:
NEVER mutate this.state directly, as calling setState() afterwards may
replace the mutation you made. Treat this.state as if it were
immutable.React.Component.
I guess, you would do it like this (not familiar with React):
var joined = this.state.myArray.concat('new value');
this.setState({ myArray: joined })
Functional Components & React Hooks
const [array,setArray] = useState([]);
Push value at the end:
setArray(oldArray => [...oldArray,newValue] );
Push value at the start:
setArray(oldArray => [newValue,...oldArray] );
Never recommended to mutate the state directly.
The recommended approach in later React versions is to use an updater function when modifying states to prevent race conditions:
Push string to end of the array
this.setState(prevState => ({
myArray: [...prevState.myArray, "new value"]
}))
Push string to beginning of the array
this.setState(prevState => ({
myArray: ["new value", ...prevState.myArray]
}))
Push object to end of the array
this.setState(prevState => ({
myArray: [...prevState.myArray, {"name": "object"}]
}))
Push object to beginning of the array
this.setState(prevState => ({
myArray: [ {"name": "object"}, ...prevState.myArray]
}))
You should not be operating the state at all. At least, not directly. If you want to update your array, you'll want to do something like this.
var newStateArray = this.state.myArray.slice();
newStateArray.push('new value');
this.setState(myArray: newStateArray);
Working on the state object directly is not desirable. You can also take a look at React's immutability helpers.
https://facebook.github.io/react/docs/update.html
Here you can not push the object to a state array like this. You can push like your way in normal array.
Here you have to set the state,
this.setState({
myArray: [...this.state.myArray, 'new value']
})
You can use .concat method to create copy of your array with new data:
this.setState({ myArray: this.state.myArray.concat('new value') })
But beware of special behaviour of .concat method when passing arrays - [1, 2].concat(['foo', 3], 'bar') will result in [1, 2, 'foo', 3, 'bar'].
Using react hooks, you can do following way
const [countryList, setCountries] = useState([]);
setCountries((countryList) => [
...countryList,
"India",
]);
This Code work for me :
fetch('http://localhost:8080')
.then(response => response.json())
.then(json => {
this.setState({mystate: this.state.mystate.push.apply(this.state.mystate, json)})
})
React-Native
if u also want ur UI (ie. ur flatList) to be up to date, use PrevState:
in the example below if user clicks on the button , it is going to add a new object to the list( both in the model and UI)
data: ['shopping','reading'] // declared in constructor
onPress={() => {this.setState((prevState, props) => {
return {data: [new obj].concat(prevState.data) };
})}}.
In the following way we can check and update the objects
this.setState(prevState => ({
Chart: this.state.Chart.length !== 0 ? [...prevState.Chart,data[data.length - 1]] : data
}));
setState([...prevState, {
label: newState.name,
value: newState.id
}]);
Was working with the dropdowns and wanted to implement this scenario there, i found this simple solution for dropdown with multiple values.
you are breaking React principles, you should clone the old state then merge it with the new data, you shouldn't manipulate your state directly,
your code should go like this
fetch('http://localhost:8080').then(response => response.json()).then(json ={this.setState({mystate[...this.state.mystate, json]}) })
If you use:
const[myArr, setMyArr] = useState([]);
for add:
setMyArr([...myArr, value]);
and for remove:
let index = myArr.indexOf(value);
if(index !== -1)
setPatch([...myArr.slice(0, index), ...myArr.slice(index, myArr.length-1)]);
I guess this is a little bit late for an answer but for those new to react
You can use this tiny package called immer
see this example: https://immerjs.github.io/immer/produce