问题
I'm creating an app where the user can design his own form. E.g. specify name of the field and details of which other columns that should be included.
The component is available as a JSFiddle here.
My initial state looks like this:
var DynamicForm = React.createClass({
getInitialState: function() {
var items = {};
items[1] = { name: 'field 1', populate_at: 'web_start',
same_as: 'customer_name',
autocomplete_from: 'customer_name', title: '' };
items[2] = { name: 'field 2', populate_at: 'web_end',
same_as: 'user_name',
autocomplete_from: 'user_name', title: '' };
return { items };
},
render: function() {
var _this = this;
return (
<div>
{ Object.keys(this.state.items).map(function (key) {
var item = _this.state.items[key];
return (
<div>
<PopulateAtCheckboxes this={this}
checked={item.populate_at} id={key}
populate_at={data.populate_at} />
</div>
);
}, this)}
<button onClick={this.newFieldEntry}>Create a new field</button>
<button onClick={this.saveAndContinue}>Save and Continue</button>
</div>
);
}
I want to update the state when the user changes any of the values, but I'm having a hard time to target the correct object:
var PopulateAtCheckboxes = React.createClass({
handleChange: function (e) {
item = this.state.items[1];
item.name = 'newName';
items[1] = item;
this.setState({items: items});
},
render: function() {
var populateAtCheckbox = this.props.populate_at.map(function(value) {
return (
<label for={value}>
<input type="radio" name={'populate_at'+this.props.id} value={value}
onChange={this.handleChange} checked={this.props.checked == value}
ref="populate-at"/>
{value}
</label>
);
}, this);
return (
<div className="populate-at-checkboxes">
{populateAtCheckbox}
</div>
);
}
});
How should I craft this.setState to get it to update items[1].name ?
回答1:
You could use the update immutability helper for this:
this.setState({
items: update(this.state.items, {1: {name: {$set: 'updated field name'}}})
})
Or if you don't care about being able to detect changes to this item in a shouldComponentUpdate() lifecycle method using ===, you could edit the state directly and force the component to re-render - this is effectively the same as @limelights' answer, as it's pulling an object out of state and editing it.
this.state.items[1].name = 'updated field name'
this.forceUpdate()
Post-edit addition:
Check out the Simple Component Communication lesson from react-training for an example of how to pass a callback function from a state-holding parent to a child component which needs to trigger a state change.
回答2:
Wrong way!
handleChange = (e) => {
const { items } = this.state;
items[1].name = e.target.value;
// update state
this.setState({
items,
});
};
As pointed out by a lot of better developers in the comments: mutating the state is wrong!
Took me a while to figure this out. Above works but it takes away the power of React. For example componentDidUpdate will not see this as an update because it's modified directly.
So the right way would be:
handleChange = (e) => {
this.setState(prevState => ({
items: {
...prevState.items,
[prevState.items[1].name]: e.target.value,
},
}));
};
回答3:
Since there's a lot of misinformation in this thread, here's how you can do it without helper libs:
handleChange: function (e) {
// 1. Make a shallow copy of the items
let items = [...this.state.items];
// 2. Make a shallow copy of the item you want to mutate
let item = {...items[1]};
// 3. Replace the property you're intested in
item.name = 'newName';
// 4. Put it back into our array. N.B. we *are* mutating the array here, but that's why we made a copy first
items[1] = item;
// 5. Set the state to our new copy
this.setState({items});
},
You can combine steps 2 and 3 if you want:
let item = {
...items[1],
name: 'newName'
}
Or you can do the whole thing in one line:
this.setState(({items}) => ({
items: [
...items.slice(0,1),
{
...items[1],
name: 'newName',
},
...items.slice(2)
]
}));
Note: I made items an array. OP used an object. However, the concepts are the same.
You can see what's going on in your terminal/console:
❯ node
> items = [{name:'foo'},{name:'bar'},{name:'baz'}]
[ { name: 'foo' }, { name: 'bar' }, { name: 'baz' } ]
> clone = [...items]
[ { name: 'foo' }, { name: 'bar' }, { name: 'baz' } ]
> item1 = {...clone[1]}
{ name: 'bar' }
> item1.name = 'bacon'
'bacon'
> clone[1] = item1
{ name: 'bacon' }
> clone
[ { name: 'foo' }, { name: 'bacon' }, { name: 'baz' } ]
> items
[ { name: 'foo' }, { name: 'bar' }, { name: 'baz' } ] // good! we didn't mutate `items`
> items === clone
false // these are different objects
> items[0] === clone[0]
true // we don't need to clone items 0 and 2 because we're not mutating them (efficiency gains!)
> items[1] === clone[1]
false // this guy we copied
回答4:
To modify deeply nested objects/variables in React's state, typically three methods are used: vanilla JavaScript's Object.assign, immutability-helper and cloneDeep from Lodash.
There are also plenty of other less popular third-party libs to achieve this, but in this answer, I'll cover just these three options. Also, some additional vanilla JavaScript methods exist, like array spreading, (see @mpen's answer for example), but they are not very intuitive, easy to use and capable to handle all state manipulation situations.
As was pointed innumerable times in top voted comments to the answers, whose authors propose a direct mutation of state: just don't do that. This is a ubiquitous React anti-pattern, which will inevitably lead to unwanted consequences. Learn the right way.
Let's compare three widely used methods.
Given this state object structure:
state = {
outer: {
inner: 'initial value'
}
}
You can use the following methods to update the inner-most inner field's value without affecting the rest of the state.
1. Vanilla JavaScript's Object.assign
const App = () => {
const [outer, setOuter] = React.useState({ inner: 'initial value' })
React.useEffect(() => {
console.log('Before the shallow copying:', outer.inner) // initial value
const newOuter = Object.assign({}, outer, { inner: 'updated value' })
console.log('After the shallow copy is taken, the value in the state is still:', outer.inner) // initial value
setOuter(newOuter)
}, [])
console.log('In render:', outer.inner)
return (
<section>Inner property: <i>{outer.inner}</i></section>
)
}
ReactDOM.render(
<App />,
document.getElementById('react')
)<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.10.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.10.2/umd/react-dom.production.min.js"></script>
<main id="react"></main>Keep in mind, that Object.assign will not perform a deep cloning, since it only copies property values, and that's why what it does is called a shallow copying (see comments).
For this to work, we should only manipulate the properties of primitive types (outer.inner), that is strings, numbers, booleans.
In this example, we're creating a new constant (const newOuter...), using Object.assign, which creates an empty object ({}), copies outer object ({ inner: 'initial value' }) into it and then copies a different object { inner: 'updated value' } over it.
This way, in the end the newly created newOuter constant will hold a value of { inner: 'updated value' } since the inner property got overridden. This newOuter is a brand new object, which is not linked to the object in state, so it can be mutated as needed and the state will stay the same and not changed until the command to update it is ran.
The last part is to use setOuter() setter to replace the original outer in the state with a newly created newOuter object (only the value will change, the property name outer will not).
Now imagine we have a more deep state like state = { outer: { inner: { innerMost: 'initial value' } } }. We could try to create the newOuter object and populate it with the outer contents from the state, but Object.assign will not be able to copy innerMost's value to this newly created newOuter object since innerMost is nested too deeply.
You could still copy inner, like in the example above, but since it's now an object and not a primitive, the reference from newOuter.inner will be copied to the outer.inner instead, which means that we will end up with local newOuter object directly tied to the object in the state.
That means that in this case mutations of the locally created newOuter.inner will directly affect the outer.inner object (in state), since they are in fact became the same thing (in computer's memory).
Object.assign therefore will only work if you have a relatively simple one level deep state structure with innermost members holding values of the primitive type.
If you have deeper objects (2nd level or more), which you should update, don't use Object.assign. You risk mutating state directly.
2. Lodash's cloneDeep
const App = () => {
const [outer, setOuter] = React.useState({ inner: 'initial value' })
React.useEffect(() => {
console.log('Before the deep cloning:', outer.inner) // initial value
const newOuter = _.cloneDeep(outer) // cloneDeep() is coming from the Lodash lib
newOuter.inner = 'updated value'
console.log('After the deeply cloned object is modified, the value in the state is still:', outer.inner) // initial value
setOuter(newOuter)
}, [])
console.log('In render:', outer.inner)
return (
<section>Inner property: <i>{outer.inner}</i></section>
)
}
ReactDOM.render(
<App />,
document.getElementById('react')
)<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.10.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.10.2/umd/react-dom.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.15/lodash.min.js"></script>
<main id="react"></main>Lodash's cloneDeep is way more simple to use. It performs a deep cloning, so it is a robust option, if you have a fairly complex state with multi-level objects or arrays inside. Just cloneDeep() the top-level state property, mutate the cloned part in whatever way you please, and setOuter() it back to the state.
3. immutability-helper
const App = () => {
const [outer, setOuter] = React.useState({ inner: 'initial value' })
React.useEffect(() => {
const update = immutabilityHelper
console.log('Before the deep cloning and updating:', outer.inner) // initial value
const newOuter = update(outer, { inner: { $set: 'updated value' } })
console.log('After the cloning and updating, the value in the state is still:', outer.inner) // initial value
setOuter(newOuter)
}, [])
console.log('In render:', outer.inner)
return (
<section>Inner property: <i>{outer.inner}</i></section>
)
}
ReactDOM.render(
<App />,
document.getElementById('react')
)<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.10.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.10.2/umd/react-dom.production.min.js"></script>
<script src="https://wzrd.in/standalone/immutability-helper@3.0.0"></script>
<main id="react"></main>immutability-helper takes it to a whole new level, and the cool thing about it is that it can not only $set values to state items, but also $push, $splice, $merge (etc.) them. Here is a list of commands available.
Side notes
Again, keep in mind, that setOuter only modifies the first-level properties of the state object (outer in these examples), not the deeply nested (outer.inner). If it behaved in a different way, this question wouldn't exist.
Which one is right for your project?
If you don't want or can use external dependencies, and have a simple state structure, stick to Object.assign.
If you manipulate a huge and/or complex state, Lodash's cloneDeep is a wise choice.
If you need advanced capabilities, i.e. if your state structure is complex and you need to perform all kinds of operations on it, try immutability-helper, it's a very advanced tool which can be used for state manipulation.
...or, do you really need to do this at all?
If you hold a complex data in React's state, maybe this is a good time to think about other ways of handling it. Setting a complex state objects right in React components is not a straightforward operation, and I strongly suggest to think about different approaches.
Most likely you better be off keeping your complex data in a Redux store, setting it there using reducers and/or sagas and access it using selectors.
回答5:
I had the same problem. Here's a simple solution that works !
const newItems = [...this.state.items];
newItems[item] = value;
this.setState({ items:newItems });
回答6:
First get the item you want, change what you want on that object and set it back on the state.
The way you're using state by only passing an object in getInitialState would be way easier if you'd use a keyed object.
handleChange: function (e) {
item = this.state.items[1];
item.name = 'newName';
items[1] = item;
this.setState({items: items});
}
回答7:
According to the React documentation on setState, using Object.assign as suggested by other answers here is not ideal. Due to the nature of setState's asynchronous behavior, subsequent calls using this technique may override previous calls causing undesirable outcomes.
Instead, the React docs recommend to use the updater form of setState which operates on the previous state. Keep in mind that when updating an array or object you must return a new array or object as React requires us to preserve state immutability. Using ES6 syntax's spread operator to shallow copy an array, creating or updating a property of an object at a given index of the array would look like this:
this.setState(prevState => {
const newItems = [...prevState.items];
newItems[index].name = newName;
return {items: newItems};
})
回答8:
Don't mutate the state in place. It can cause unexpected results. I have learned my lesson! Always work with a copy/clone, Object.assign() is a good one:
item = Object.assign({}, this.state.items[1], {name: 'newName'});
items[1] = item;
this.setState({items: items});
https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_Objects/Object/assign
回答9:
It's really simple.
First pull the entire items object from state, updated the part of the items object as desired, and put the entire items object back in state via setState.
handleChange: function (e) {
items = Object.assign(this.state.items); // Pull the entire items object out. Using object.assign is a good idea for objects.
items[1].name = 'newName'; // update the items object as needed
this.setState({ items }); // Put back in state
}
回答10:
Mutation free:
// given a state
state = {items: [{name: 'Fred', value: 1}, {name: 'Wilma', value: 2}]}
// This will work without mutation as it clones the modified item in the map:
this.state.items
.map(item => item.name === 'Fred' ? {...item, ...{value: 3}} : item)
this.setState(newItems)
回答11:
Use the event on handleChange to figure out the element that has changed and then update it. For that you might need to change some property to identify it and update it.
See fiddle https://jsfiddle.net/69z2wepo/6164/
回答12:
I would move the function handle change and add an index parameter
handleChange: function (index) {
var items = this.state.items;
items[index].name = 'newName';
this.setState({items: items});
},
to the Dynamic form component and pass it to the PopulateAtCheckboxes component as a prop. As you loop over your items you can include an additional counter (called index in the code below) to be passed along to the handle change as shown below
{ Object.keys(this.state.items).map(function (key, index) {
var item = _this.state.items[key];
var boundHandleChange = _this.handleChange.bind(_this, index);
return (
<div>
<PopulateAtCheckboxes this={this}
checked={item.populate_at} id={key}
handleChange={boundHandleChange}
populate_at={data.populate_at} />
</div>
);
}, this)}
Finally you can call your change listener as shown below here
<input type="radio" name={'populate_at'+this.props.id} value={value} onChange={this.props.handleChange} checked={this.props.checked == value} ref="populate-at"/>
回答13:
If you need to change only part of the Array,
You've a react component with state set to.
state = {items: [{name: 'red-one', value: 100}, {name: 'green-one', value: 999}]}
It's best to update the red-one in the Array as follows:
const itemIndex = this.state.items.findIndex(i=> i.name === 'red-one');
const newItems = [
this.state.items.slice(0, itemIndex),
{name: 'red-one', value: 666},
this.state.items.slice(itemIndex)
]
this.setState(newItems)
回答14:
As none of the above options was ideal to me I ended up using map:
this.setState({items: this.state.items.map((item,idx)=> idx!==1 ?item :{...item,name:'new_name'}) })
回答15:
or if you have a dynamically generated list and you don't know the index but just have the key or id:
let ItemsCopy = []
let x = this.state.Items.map((entry) =>{
if(entry.id == 'theIDYoureLookingFor')
{
entry.PropertyToChange = 'NewProperty'
}
ItemsCopy.push(entry)
})
this.setState({Items:ItemsCopy});
回答16:
Try with code:
this.state.items[1] = 'new value';
var cloneObj = Object.assign({}, this.state.items);
this.setState({items: cloneObj });
回答17:
Following piece of code went easy on my dull brain. Removing the object and replacing with the updated one
var udpateditem = this.state.items.find(function(item) {
return item.name == "field_1" });
udpateditem.name= "New updated name"
this.setState(prevState => ({
items:prevState.dl_name_template.filter(function(item) {
return item.name !== "field_1"}).concat(udpateditem)
}));
回答18:
How about creating another component(for object that needs to go into the array) and pass the following as props?
- component index - index will be used to create/update in array.
- set function - This function put data into the array based on the component index.
<SubObjectForm setData={this.setSubObjectData} objectIndex={index}/>
Here {index} can be passed in based on position where this SubObjectForm is used.
and setSubObjectData can be something like this.
setSubObjectData: function(index, data){
var arrayFromParentObject= <retrieve from props or state>;
var objectInArray= arrayFromParentObject.array[index];
arrayFromParentObject.array[index] = Object.assign(objectInArray, data);
}
In SubObjectForm, this.props.setData can be called on data change as given below.
<input type="text" name="name" onChange={(e) => this.props.setData(this.props.objectIndex,{name: e.target.value})}/>
回答19:
Try this it will definetly work,other case i tried but didn't work
import _ from 'lodash';
this.state.var_name = _.assign(this.state.var_name, {
obj_prop: 'changed_value',
});
来源：https://stackoverflow.com/questions/29537299/react-how-to-update-state-item1-in-state-using-setstate