I'm trying to organize my state by using nested property like this:
this.state = {
someProperty: {
flag:true
}
}
But updating state like this,
this.setState({ someProperty.flag: false });
doesn't work. How can this be done correctly?
In order to setState for a nested object you can follow the below approach as I think setState doesn't handle nested updates.
var someProperty = {...this.state.someProperty}
someProperty.flag = true;
this.setState({someProperty})
The idea is to create a dummy object perform operations on it and then replace the component's state with the updated object
Now, the spread operator creates only one level nested copy of the object. If your state is highly nested like:
this.state = {
someProperty: {
someOtherProperty: {
anotherProperty: {
flag: true
}
..
}
...
}
...
}
You could setState using spread operator at each level like
this.setState(prevState => ({
...prevState,
someProperty: {
...prevState.someProperty,
someOtherProperty: {
...prevState.someProperty.someOtherProperty,
anotherProperty: {
...prevState.someProperty.someOtherProperty.anotherProperty,
flag: false
}
}
}
}))
However the above syntax get every ugly as the state becomes more and more nested and hence I recommend you to use immutability-helper package to update the state.
See this answer on how to update state with immutability-helper.
To write it in one line
this.setState({ someProperty: { ...this.state.someProperty, flag: false} });
Sometimes direct answers are not the best ones :)
Short version:
this code
this.state = {
someProperty: {
flag: true
}
}
should be simplified as something like
this.state = {
somePropertyFlag: true
}
Long version:
Currently you shouldn't want to work with nested state in React. Because React is not oriented to work with nested states and all solutions proposed here look as hacks. They don't use the framework but fight with it. They suggest to write not so clear code for doubtful purpose of grouping some properties. So they are very interesting as an answer to the challenge but practically useless.
Lets imagine the following state:
{
parent: {
child1: 'value 1',
child2: 'value 2',
...
child100: 'value 100'
}
}
What will happen if you change just a value of child1? React will not re-render the view because it uses shallow comparison and it will find that parent property didn't change. BTW mutating the state object directly is considered to be a bad practice in general.
So you need to re-create the whole parent object. But in this case we will meet another problem. React will think that all children have changed their values and will re-render all of them. Of course it is not good for performance.
It is still possible to solve that problem by writing some complicated logic in shouldComponentUpdate() but I would prefer to stop here and use simple solution from the short version.
Disclaimer
Nested State in React is wrong design
Read this excellent answer.
Reasoning behind this answer:
React's setState is just a built-in convenience, but you soon realise
that it has its limits. Using custom properties and intelligent use of
forceUpdate gives you much more.
eg:
class MyClass extends React.Component {
myState = someObject
inputValue = 42
...
MobX, for example, ditches state completely and uses custom observable properties.
Use Observables instead of state in React components.
the answer to your misery - see example here
There is another shorter way to update whatever nested property.
this.setState(state => {
state.nested.flag = false
state.another.deep.prop = true
return state
})
On one line
this.setState(state => (state.nested.flag = false, state))
note: This here is Comma operator ~MDN, see it in action here (Sandbox).
It is similar to (though this doesn't change state reference)
this.state.nested.flag = false
this.forceUpdate()
For the subtle difference in this context between forceUpdate and setState see the linked example and sandbox.
Of course this is abusing some core principles, as the state should be read-only, but since you are immediately discarding the old state and replacing it with new state, it is completely ok.
Warning
Even though the component containing the state will update and rerender properly (except this gotcha), the props will fail to propagate to children (see Spymaster's comment below). Only use this technique if you know what you are doing.
For example, you may pass a changed flat prop that is updated and passed easily.
render(
//some complex render with your nested state
<ChildComponent complexNestedProp={this.state.nested} pleaseRerender={Math.random()}/>
)
Now even though reference for complexNestedProp did not change (shouldComponentUpdate)
this.props.complexNestedProp === nextProps.complexNestedProp
the component will rerender whenever parent component updates, which is the case after calling this.setState or this.forceUpdate in the parent.
Effects of mutating the state sandbox
Using nested state and mutating the state directly is dangerous because different objects might hold (intentionally or not) different (older) references to the state and might not necessarily know when to update (for example when using PureComponent or if shouldComponentUpdate is implemented to return false) OR are intended to display old data like in the example below.
Imagine a timeline that is supposed to render historic data, mutating the data under the hand will result in unexpected behaviour as it will also change previous items.
Anyway here you can see that Nested PureChildClass is not rerendered due to props failing to propagate.
const newState = Object.assign({}, this.state);
newState.property.nestedProperty = "new value";
this.setState(newState);
If you are using ES2015 you have access to the Object.assign. You can use it as follows to update a nested object.
this.setState({
someProperty: Object.assign({}, this.state.someProperty, {flag: false})
});
You merge the updated properties with the existing and use the returned object to update the state.
Edit: Added an empty object as target to the assign function to make sure the state isn't mutated directly as carkod pointed out.
We use Immer https://github.com/mweststrate/immer to handle these kinds of issues.
Just replaced this code in one of our components
this.setState(prevState => ({
...prevState,
preferences: {
...prevState.preferences,
[key]: newValue
}
}));
With this
import produce from 'immer';
this.setState(produce(draft => {
draft.preferences[key] = newValue;
}));
With immer you handle your state as a "normal object".
The magic happens behind the scene with proxy objects.
There are many libraries to help with this. For example, using immutability-helper:
import update from 'immutability-helper';
const newState = update(this.state, {
someProperty: {flag: {$set: false}},
};
this.setState(newState);
Using lodash/fp set:
import {set} from 'lodash/fp';
const newState = set(["someProperty", "flag"], false, this.state);
Using lodash/fp merge:
import {merge} from 'lodash/fp';
const newState = merge(this.state, {
someProperty: {flag: false},
});
Although you asked about a state of class-based React component, the same problem exists with useState hook. Even worse: useState hook does not accept partial updates. So this question became very relevant when useState hook was introduced.
I have decided to post the following answer to make sure the question covers more modern scenarios where the useState hook is used:
If you have:
const [state, setState] = useState({
someProperty: {
flag: true,
otherNestedProp: 1
},
otherProp: 2
})
you can set the nested property by cloning the current and patching the required segments of the data, for example:
setState(current => { ...current,
someProperty: { ...current.someProperty,
flag: false
}
});
Or you can use Immer library to simplify the cloning and patching of the object.
Or you can use Hookstate library (disclaimer: I am an author) to simply the management of complex (local and global) state data entirely and improve the performance (read: not to worry about rendering optimization):
import { useHookstate } from '#hookstate/core'
const state = useHookstate({
someProperty: {
flag: true,
otherNestedProp: 1
},
otherProp: 2
})
get the field to render:
state.someProperty.flag.get()
// or
state.get().someProperty.flag
set the nested field:
state.someProperty.flag.set(false)
Here is the Hookstate example, where the state is deeply / recursively nested in tree-like data structure.
Here's a variation on the first answer given in this thread which doesn't require any extra packages, libraries or special functions.
state = {
someProperty: {
flag: 'string'
}
}
handleChange = (value) => {
const newState = {...this.state.someProperty, flag: value}
this.setState({ someProperty: newState })
}
In order to set the state of a specific nested field, you have set the whole object. I did this by creating a variable, newState and spreading the contents of the current state into it first using the ES2015 spread operator. Then, I replaced the value of this.state.flag with the new value (since I set flag: value after I spread the current state into the object, the flag field in the current state is overridden). Then, I simply set the state of someProperty to my newState object.
I used this solution.
If you have a nested state like this:
this.state = {
formInputs:{
friendName:{
value:'',
isValid:false,
errorMsg:''
},
friendEmail:{
value:'',
isValid:false,
errorMsg:''
}
}
you can declare the handleChange function that copy current status and re-assigns it with changed values
handleChange(el) {
let inputName = el.target.name;
let inputValue = el.target.value;
let statusCopy = Object.assign({}, this.state);
statusCopy.formInputs[inputName].value = inputValue;
this.setState(statusCopy);
}
here the html with the event listener
<input type="text" onChange={this.handleChange} " name="friendName" />
Although nesting isn't really how you should treat a component state, sometimes for something easy for single tier nesting.
For a state like this
state = {
contact: {
phone: '888-888-8888',
email: 'test#test.com'
}
address: {
street:''
},
occupation: {
}
}
A re-useable method ive used would look like this.
handleChange = (obj) => e => {
let x = this.state[obj];
x[e.target.name] = e.target.value;
this.setState({ [obj]: x });
};
then just passing in the obj name for each nesting you want to address...
<TextField
name="street"
onChange={handleChange('address')}
/>
Create a copy of the state:
let someProperty = JSON.parse(JSON.stringify(this.state.someProperty))
make changes in this object:
someProperty.flag = "false"
now update the state
this.setState({someProperty})
Not sure if this is technically correct according to the framework's standards, but sometimes you simply need to update nested objects. Here is my solution using hooks.
setInputState({
...inputState,
[parentKey]: { ...inputState[parentKey], [childKey]: value },
});
I am seeing everyone has given the class based component state update solve which is expected because he asked that for but I am trying to give the same solution for hook.
const [state, setState] = useState({
state1: false,
state2: 'lorem ipsum'
})
Now if you want to change the nested object key state1 only then you can do the any of the following:
Process 1
let oldState = state;
oldState.state1 = true
setState({...oldState);
Process 2
setState(prevState => ({
...prevState,
state1: true
}))
I prefer the process 2 most.
Two other options not mentioned yet:
If you have deeply nested state, consider if you can restructure the child objects to sit at the root. This makes the data easier to update.
There are many handy libraries available for handling immutable state listed in the Redux docs. I recommend Immer since it allows you to write code in a mutative manner but handles the necessary cloning behind the scenes. It also freezes the resulting object so you can't accidentally mutate it later.
To make things generic, I worked on #ShubhamKhatri's and #Qwerty's answers.
state object
this.state = {
name: '',
grandParent: {
parent1: {
child: ''
},
parent2: {
child: ''
}
}
};
input controls
<input
value={this.state.name}
onChange={this.updateState}
type="text"
name="name"
/>
<input
value={this.state.grandParent.parent1.child}
onChange={this.updateState}
type="text"
name="grandParent.parent1.child"
/>
<input
value={this.state.grandParent.parent2.child}
onChange={this.updateState}
type="text"
name="grandParent.parent2.child"
/>
updateState method
setState as #ShubhamKhatri's answer
updateState(event) {
const path = event.target.name.split('.');
const depth = path.length;
const oldstate = this.state;
const newstate = { ...oldstate };
let newStateLevel = newstate;
let oldStateLevel = oldstate;
for (let i = 0; i < depth; i += 1) {
if (i === depth - 1) {
newStateLevel[path[i]] = event.target.value;
} else {
newStateLevel[path[i]] = { ...oldStateLevel[path[i]] };
oldStateLevel = oldStateLevel[path[i]];
newStateLevel = newStateLevel[path[i]];
}
}
this.setState(newstate);
}
setState as #Qwerty's answer
updateState(event) {
const path = event.target.name.split('.');
const depth = path.length;
const state = { ...this.state };
let ref = state;
for (let i = 0; i < depth; i += 1) {
if (i === depth - 1) {
ref[path[i]] = event.target.value;
} else {
ref = ref[path[i]];
}
}
this.setState(state);
}
Note: These above methods won't work for arrays
I take very seriously the concerns already voiced around creating a complete copy of your component state. With that said, I would strongly suggest Immer.
import produce from 'immer';
<Input
value={this.state.form.username}
onChange={e => produce(this.state, s => { s.form.username = e.target.value }) } />
This should work for React.PureComponent (i.e. shallow state comparisons by React) as Immer cleverly uses a proxy object to efficiently copy an arbitrarily deep state tree. Immer is also more typesafe compared to libraries like Immutability Helper, and is ideal for Javascript and Typescript users alike.
Typescript utility function
function setStateDeep<S>(comp: React.Component<any, S, any>, fn: (s:
Draft<Readonly<S>>) => any) {
comp.setState(produce(comp.state, s => { fn(s); }))
}
onChange={e => setStateDeep(this, s => s.form.username = e.target.value)}
setInputState((pre)=> ({...pre,[parentKey]: {...pre[parentKey], [childKey]: value}}));
I'd like this
If you want to set the state dynamically
following example sets the state of form dynamically where each key in state is object
onChange(e:React.ChangeEvent<HTMLInputElement | HTMLTextAreaElement>) {
this.setState({ [e.target.name]: { ...this.state[e.target.name], value: e.target.value } });
}
I found this to work for me, having a project form in my case where for example you have an id, and a name and I'd rather maintain state for a nested project.
return (
<div>
<h2>Project Details</h2>
<form>
<Input label="ID" group type="number" value={this.state.project.id} onChange={(event) => this.setState({ project: {...this.state.project, id: event.target.value}})} />
<Input label="Name" group type="text" value={this.state.project.name} onChange={(event) => this.setState({ project: {...this.state.project, name: event.target.value}})} />
</form>
</div>
)
Let me know!
stateUpdate = () => {
let obj = this.state;
if(this.props.v12_data.values.email) {
obj.obj_v12.Customer.EmailAddress = this.props.v12_data.values.email
}
this.setState(obj)
}
This is clearly not the right or best way to do, however it is cleaner to my view:
this.state.hugeNestedObject = hugeNestedObject;
this.state.anotherHugeNestedObject = anotherHugeNestedObject;
this.setState({})
However, React itself should iterate thought nested objects and update state and DOM accordingly which is not there yet.
Use this for multiple input control and dynamic nested name
<input type="text" name="title" placeholder="add title" onChange={this.handleInputChange} />
<input type="checkbox" name="chkusein" onChange={this.handleInputChange} />
<textarea name="body" id="" cols="30" rows="10" placeholder="add blog content" onChange={this.handleInputChange}></textarea>
the code very readable
the handler
handleInputChange = (event) => {
const target = event.target;
const value = target.type === 'checkbox' ? target.checked : target.value;
const name = target.name;
const newState = { ...this.state.someProperty, [name]: value }
this.setState({ someProperty: newState })
}
Here's a full example using nested state (one level) with the solution in this answer, for a component implemented as a class:
class CaveEditModal extends React.Component {
// ...
constructor(props, context) {
super(props);
this.state = {
tabValue: '1',
isModalOpen: this.props.isModalOpen,
// ...
caveData: {
latitude: 1,
longitude: 2
}
};
// ...
const updateNestedFieldEvent = fullKey => ev => {
var [parentProperty, _key] = fullKey.split(".", 2);
this.setState({[parentProperty]: { ...this.state[parentProperty], [_key]: ev.target.value} });
};
// ...
this.handleLatitudeChange = updateNestedFieldEvent('caveData.latitude');
this.handleLongitudeChange = updateNestedFieldEvent('caveData.longitude');
}
render () {
return (
<div>
<TextField id="latitude" label="Latitude" type="number" value={this.state.caveData.latitude} onChange={this.handleLatitudeChange} />
<TextField id="longitude" label="Longitude" type="number" value={this.state.caveData.longitude} onChange={this.handleLongitudeChange} />
<span>lat={this.state.caveData.latitude} long={this.state.caveData.longitude}</span>
</div>
);
};
}
Note that the state updater function updateNestedFieldEvent works only for one level nested object like a.b, not like a.b.c.
For someone who read in 2022:
constructor(props) {
super(props);
this.state = {
someProperty: {
flag: true
}
otherValues: '',
errors: {}
};
this.handleInputChange = this.handleInputChange.bind(this);
}
handleInputChange(event) {
const target = event.target;
const value = target.type === 'checkbox' ? target.checked : target.value;
const name = target.name;
const someProperty = { ...this.state.someProperty };
someProperty[name] = value;
this.setState({
someProperty: someProperty
});
}
.......
Use arrow function instead, this should do the trick.
setItems((prevState) => {
prevState.nestedData = newNestedData;
prevState.nestedData1 = newNestedData1;
});
don't forget to use the arrow function (prevState) => {update js assignment statements...}
Something like this might suffice,
const isObject = (thing) => {
if(thing &&
typeof thing === 'object' &&
typeof thing !== null
&& !(Array.isArray(thing))
){
return true;
}
return false;
}
/*
Call with an array containing the path to the property you want to access
And the current component/redux state.
For example if we want to update `hello` within the following obj
const obj = {
somePrimitive:false,
someNestedObj:{
hello:1
}
}
we would do :
//clone the object
const cloned = clone(['someNestedObj','hello'],obj)
//Set the new value
cloned.someNestedObj.hello = 5;
*/
const clone = (arr, state) => {
let clonedObj = {...state}
const originalObj = clonedObj;
arr.forEach(property => {
if(!(property in clonedObj)){
throw new Error('State missing property')
}
if(isObject(clonedObj[property])){
clonedObj[property] = {...originalObj[property]};
clonedObj = clonedObj[property];
}
})
return originalObj;
}
const nestedObj = {
someProperty:true,
someNestedObj:{
someOtherProperty:true
}
}
const clonedObj = clone(['someProperty'], nestedObj);
console.log(clonedObj === nestedObj) //returns false
console.log(clonedObj.someProperty === nestedObj.someProperty) //returns true
console.log(clonedObj.someNestedObj === nestedObj.someNestedObj) //returns true
console.log()
const clonedObj2 = clone(['someProperty','someNestedObj','someOtherProperty'], nestedObj);
console.log(clonedObj2 === nestedObj) // returns false
console.log(clonedObj2.someNestedObj === nestedObj.someNestedObj) //returns false
//returns true (doesn't attempt to clone because its primitive type)
console.log(clonedObj2.someNestedObj.someOtherProperty === nestedObj.someNestedObj.someOtherProperty)
I know it is an old question but still wanted to share how i achieved this. Assuming state in constructor looks like this:
constructor(props) {
super(props);
this.state = {
loading: false,
user: {
email: ""
},
organization: {
name: ""
}
};
this.handleChange = this.handleChange.bind(this);
}
My handleChange function is like this:
handleChange(e) {
const names = e.target.name.split(".");
const value = e.target.type === "checkbox" ? e.target.checked : e.target.value;
this.setState((state) => {
state[names[0]][names[1]] = value;
return {[names[0]]: state[names[0]]};
});
}
And make sure you name inputs accordingly:
<input
type="text"
name="user.email"
onChange={this.handleChange}
value={this.state.user.firstName}
placeholder="Email Address"
/>
<input
type="text"
name="organization.name"
onChange={this.handleChange}
value={this.state.organization.name}
placeholder="Organization Name"
/>
I do nested updates with a reduce search:
Example:
The nested variables in state:
state = {
coords: {
x: 0,
y: 0,
z: 0
}
}
The function:
handleChange = nestedAttr => event => {
const { target: { value } } = event;
const attrs = nestedAttr.split('.');
let stateVar = this.state[attrs[0]];
if(attrs.length>1)
attrs.reduce((a,b,index,arr)=>{
if(index==arr.length-1)
a[b] = value;
else if(a[b]!=null)
return a[b]
else
return a;
},stateVar);
else
stateVar = value;
this.setState({[attrs[0]]: stateVar})
}
Use:
<input
value={this.state.coords.x}
onChange={this.handleTextChange('coords.x')}
/>
I'm finding these two pieces of the React Hooks docs a little confusing. Which one is the best practice for updating a state object using the state hook?
Imagine a want to make the following state update:
INITIAL_STATE = {
propA: true,
propB: true
}
stateAfter = {
propA: true,
propB: false // Changing this property
}
OPTION 1
From the Using the React Hook article, we get that this is possible:
const [count, setCount] = useState(0);
setCount(count + 1);
So I could do:
const [myState, setMyState] = useState(INITIAL_STATE);
And then:
setMyState({
...myState,
propB: false
});
OPTION 2
And from the Hooks Reference we get that:
Unlike the setState method found in class components, useState does
not automatically merge update objects. You can replicate this
behavior by combining the function updater form with object spread
syntax:
setState(prevState => {
// Object.assign would also work
return {...prevState, ...updatedValues};
});
As far as I know, both works. So, what is the difference? Which one is the best practice? Should I use pass the function (OPTION 2) to access the previous state, or should I simply access the current state with spread syntax (OPTION 1)?
Both options are valid, but just as with setState in a class component you need to be careful when updating state derived from something that already is in state.
If you e.g. update a count twice in a row, it will not work as expected if you don't use the function version of updating the state.
const { useState } = React;
function App() {
const [count, setCount] = useState(0);
function brokenIncrement() {
setCount(count + 1);
setCount(count + 1);
}
function increment() {
setCount(count => count + 1);
setCount(count => count + 1);
}
return (
<div>
<div>{count}</div>
<button onClick={brokenIncrement}>Broken increment</button>
<button onClick={increment}>Increment</button>
</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>
If anyone is searching for useState() hooks update for object
Through Input
const [state, setState] = useState({ fName: "", lName: "" });
const handleChange = e => {
const { name, value } = e.target;
setState(prevState => ({
...prevState,
[name]: value
}));
};
<input
value={state.fName}
type="text"
onChange={handleChange}
name="fName"
/>
<input
value={state.lName}
type="text"
onChange={handleChange}
name="lName"
/>
Through onSubmit or button click
setState(prevState => ({
...prevState,
fName: 'your updated value here'
}));
The best practice is to use separate calls:
const [a, setA] = useState(true);
const [b, setB] = useState(true);
Option 1 might lead to more bugs because such code often end up inside a closure which has an outdated value of myState.
Option 2 should be used when the new state is based on the old one:
setCount(count => count + 1);
For complex state structure consider using useReducer
For complex structures that share some shape and logic you can create a custom hook:
function useField(defaultValue) {
const [value, setValue] = useState(defaultValue);
const [dirty, setDirty] = useState(false);
const [touched, setTouched] = useState(false);
function handleChange(e) {
setValue(e.target.value);
setTouched(true);
}
return {
value, setValue,
dirty, setDirty,
touched, setTouched,
handleChange
}
}
function MyComponent() {
const username = useField('some username');
const email = useField('some#mail.com');
return <input name="username" value={username.value} onChange={username.handleChange}/>;
}
Which one is the best practice for updating a state object using the state hook?
They are both valid as other answers have pointed out.
what is the difference?
It seems like the confusion is due to "Unlike the setState method found in class components, useState does not automatically merge update objects", especially the "merge" part.
Let's compare this.setState & useState
class SetStateApp extends React.Component {
state = {
propA: true,
propB: true
};
toggle = e => {
const { name } = e.target;
this.setState(
prevState => ({
[name]: !prevState[name]
}),
() => console.log(`this.state`, this.state)
);
};
...
}
function HooksApp() {
const INITIAL_STATE = { propA: true, propB: true };
const [myState, setMyState] = React.useState(INITIAL_STATE);
const { propA, propB } = myState;
function toggle(e) {
const { name } = e.target;
setMyState({ [name]: !myState[name] });
}
...
}
Both of them toggles propA/B in toggle handler.
And they both update just one prop passed as e.target.name.
Check out the difference it makes when you update just one property in setMyState.
Following demo shows that clicking on propA throws an error(which occurs setMyState only),
You can following along
Warning: A component is changing a controlled input of type checkbox to be uncontrolled. Input elements should not switch from controlled to uncontrolled (or vice versa). Decide between using a controlled or uncontrolled input element for the lifetime of the component.
It's because when you click on propA checkbox, propB value is dropped and only propA value is toggled thus making propB's checked value as undefined making the checkbox uncontrolled.
And the this.setState updates only one property at a time but it merges other property thus the checkboxes stay controlled.
I dug thru the source code and the behavior is due to useState calling useReducer
Internally, useState calls useReducer, which returns whatever state a reducer returns.
https://github.com/facebook/react/blob/2b93d686e3/packages/react-reconciler/src/ReactFiberHooks.js#L1230
useState<S>(
initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
currentHookNameInDev = 'useState';
...
try {
return updateState(initialState);
} finally {
...
}
},
where updateState is the internal implementation for useReducer.
function updateState<S>(
initialState: (() => S) | S,
): [S, Dispatch<BasicStateAction<S>>] {
return updateReducer(basicStateReducer, (initialState: any));
}
useReducer<S, I, A>(
reducer: (S, A) => S,
initialArg: I,
init?: I => S,
): [S, Dispatch<A>] {
currentHookNameInDev = 'useReducer';
updateHookTypesDev();
const prevDispatcher = ReactCurrentDispatcher.current;
ReactCurrentDispatcher.current = InvalidNestedHooksDispatcherOnUpdateInDEV;
try {
return updateReducer(reducer, initialArg, init);
} finally {
ReactCurrentDispatcher.current = prevDispatcher;
}
},
If you are familiar with Redux, you normally return a new object by spreading over previous state as you did in option 1.
setMyState({
...myState,
propB: false
});
So if you set just one property, other properties are not merged.
One or more options regarding state type can be suitable depending on your usecase
Generally you could follow the following rules to decide the sort of state that you want
First: Are the individual states related
If the individual state that you have in your application are related to one other then you can choose to group them together in an object. Else its better to keep them separate and use multiple useState so that when dealing with specific handlers you are only updating the relavant state property and are not concerned about the others
For instance, user properties such as name, email are related and you can group them together Whereas for maintaining multiple counters you can make use of multiple useState hooks
Second: Is the logic to update state complex and depends on the handler or user interaction
In the above case its better to make use of useReducer for state definition. Such kind of scenario is very common when you are trying to create for example and todo app where you want to update, create and delete elements on different interactions
Should I use pass the function (OPTION 2) to access the previous
state, or should I simply access the current state with spread syntax
(OPTION 1)?
state updates using hooks are also batched and hence whenever you want to update state based on previous one its better to use the callback pattern.
The callback pattern to update state also comes in handy when the setter doesn't receive updated value from enclosed closure due to it being defined only once. An example of such as case if the useEffect being called only on initial render when adds a listener that updates state on an event.
Both are perfectly fine for that use case. The functional argument that you pass to setState is only really useful when you want to conditionally set the state by diffing the previous state (I mean you can just do it with logic surrounding the call to setState but I think it looks cleaner in the function) or if you set state in a closure that doesn't have immediate access to the freshest version of previous state.
An example being something like an event listener that is only bound once (for whatever reason) on mount to the window. E.g.
useEffect(function() {
window.addEventListener("click", handleClick)
}, [])
function handleClick() {
setState(prevState => ({...prevState, new: true }))
}
If handleClick was only setting the state using option 1, it would look like setState({...prevState, new: true }). However, this would likely introduce a bug because prevState would only capture the state on initial render and not from any updates. The function argument passed to setState would always have access to the most recent iteration of your state.
Both options are valid but they do make a difference.
Use Option 1 (setCount(count + 1)) if
Property doesn't matter visually when it updates browser
Sacrifice refresh rate for performance
Updating input state based on event (ie event.target.value); if you use Option 2, it will set event to null due to performance reasons unless you have event.persist() - Refer to event pooling.
Use Option 2 (setCount(c => c + 1)) if
Property does matter when it updates on the browser
Sacrifice performance for better refresh rate
I noticed this issue when some Alerts with autoclose feature that should close sequentially closed in batches.
Note: I don't have stats proving the difference in performance but its based on a React conference on React 16 performance optimizations.
I find it very convenient to use useReducer hook for managing complex state, instead of useState. You initialize state and updating function like this:
const initialState = { name: "Bob", occupation: "builder" };
const [state, updateState] = useReducer(
(state, updates) => {...state, ...updates},
initialState
);
And then you're able to update your state by only passing partial updates:
updateState({ occupation: "postman" })
The solution I am going to propose is much simpler and easier to not mess up than the ones above, and has the same usage as the useState API.
Use the npm package use-merge-state (here). Add it to your dependencies, then, use it like:
const useMergeState = require("use-merge-state") // Import
const [state, setState] = useMergeState(initial_state, {merge: true}) // Declare
setState(new_state) // Just like you set a new state with 'useState'
Hope this helps everyone. :)
I know that similar questions were asked before, but what if i don't want to set the entire state, only one of its properties, to a variable? Something like this:
var initialProperty = {
name: '',
id: 0,
type: ''
}
class Example extends React.Component{
constructor(props){
super(props);
this.state = {
otherProperty: '',
targetProperty: initialProperty
//at the start of the Component, everything works fine.
//the targetproperty is set to initialproperty
}
}
//by the time, the targetproperty was changed from the initial in the component
//but if i click a button, i want to set targetproperty to the initialproperty
somethingHappensOnClick = () => {
this.setState({targetProperty: initialProperty})
//unfortunately, the targetproperty wasn't set to the initial.
}
}
Am I doing something wrong? Why targetProperty doesn't change?
This is happening because, in js array and object get copied by reference. So when you are setting
targetProperty: initialProperty
targetProperty will get the reference of initialProperty, and all the changes that you will do to targetProperty, will get applied to initialProperty as well.
Idea is, create a new object each time, instead of copying the reference.
Write it like this:
var initialProperty = {
name: '',
id: 0,
type: ''
}
class Example extendds React.Component{
constructor(props){
super(props);
this.state = {
otherProperty: '',
targetProperty: Object.assign({}, initialProperty) // or {...initialProperty}
}
}
somethingHappensOnClick = () => {
this.setState({targetProperty: Object.assign({}, initialProperty)})
}
}
When you are setting targetProperty : initialProperty what happens is
initialProperty--->some_memory_location_x
//after doing targetProperty: initialProperty
targetProperty---->some_memory_location_x
so when you are mutating the targetProperty you are actually changing the values in the memory some_memory_location_x which is where your initialProperty is also pointing at so when you setState again, your targetProperty value doesn't change so try doing the way #Mayank Shukla Pointed out so that you don't mutate values
I have a component with two functions which should update state object:
class Categories extends Component {
constructor(props) {
super(props);
this.state = {
data: [],
categoryData: [],
objects: [],
object:[],
};
}
componentDidMount() {
this.setState({
data:data.Dluga,
categoryData: data.Dluga.basic,
objects:data,
})
}
changeCategory(event) {
event.preventDefault();
this.setState({
categoryData: this.state.data[(event.currentTarget.textContent).split(' ')[1]],
});
}
changeObject(event) {
event.preventDefault();
const objectOne = Object.assign({}, this.state.objects[event.currentTarget.parentElement.parentElement.children[0].children[0].value]);
this.setState({
objects: this.state.objects,
object:objectOne,
});
};
render() {
return (
<div className='categories'>
<SelectObejct onValueChange={this.changeObject}/>
<ul>
{Object.keys(this.state.data).map((item) => {
return (
<li className='category' key={item}
onClick={this.changeCategory.bind(this)}>
<span className='category-item'> {item}</span>
</li>
)})
}
</ul>
<div>
<CategoryData categoryData={this.state.categoryData}/>
</div>
</div>
);
}
}
When I update state with changeObject I have in state object two properties: objects and object, but initially it was 4 properties... Next when I update state with changeCategory I have initial properties from componentDidMount and updated categoryData but object is empty... I can't update state in one function because it's two onClick elements. What should I do to update state correctly?
The primary thing you're doing incorrectly is updating state based on existing state without using the callback version of setState. State updates can be asynchronous, and can be combined (batched). Any time you're setting state derived from the current state, you must use the callback form. E.g.:
changeCategory(event) {
event.preventDefault();
this.setState(prevState = > {
return {
categoryData: prevState.data[(event.currentTarget.textContent).split(' ')[1]]
};
});
}
Note that we're passing it a function, which will get called later (only a tiny bit later, but later), and will get the then-current state passed to it as a parameter; and we return the new state as a return value.
When I update state with changeObject I have in state object two properties: objects and object, but initially it was 4 properties...
That's absolutely normal. It's common to only specify a subset of your state properties when calling setState. In fact, changeObject should be:
changeObject(event) {
event.preventDefault();
this.setState(prevState => {
const objectOne = Object.assign({}, prevState.objects[event.currentTarget.parentElement.parentElement.children[0].children[0].value]);
return { object: objectOne };
});
}
Note that I didn't specify objects: prevState.objects. There's no reason to if you're not changing it.
Next when I update state with changeCategory I have initial properties from componentDidMount and updated categoryData but object is empty.
object will only be empty (whatever "empty" means) if you set it to that at some point. I suspect resolving the above will resolve this issue, but if not, and if you can't figure it out with further debugging, I suggest posting a new question with an [mcve] demonstrating that problem (you can do a runnable one with Stack Snippets; here's how).