Using class properties in React render() - reactjs

Question

Consider this example:
class App extends Component {
constructor() {
super();
this.person = new Person("Tim", 23);
this.state = {
name: this.person.name
}
}
changeName() {
this.person.setName("Jane");
this.person.setAge(22);
setState({name: this.person.name});
}
render() {
return (
<div>
<div>Your name is: {this.state.name}</div>
<div>Your age is: {this.person.age}</div>
<div><button onClick={this.changeName.bind(this)}>Change Name</button></div>
</div>
)
}
}
What I'm querying here is when a variable should be added to the state. In this example, although this works age isn't in the state.
I run into this a lot when working with objects, I'm not sure if it's best practise to add any rendered object property to the state, or if I should only worry about adding properties to the state if they're potentially going to be updated. I'm quite sure what I'm doing in this example would be bad, as age is updating, but isn't being reflected in the state.
Any ideas on the "correct" way to do this?

Answer 1

React doesn't dictate how you manage your data. If you're using an object with getters/setters, then it might be simpler to store the entire object in state:
changeName() {
this.person.setName("Jane");
this.person.setAge(22);
this.setState({person: this.person});
}
In this manner, your object continues to be responsible for the data, and whatever internal processing this implies, while the resultant object itself is stored in the component state.
That said, using data objects like Person, while possible, is not idiomatic React. I would recommend using something like Redux, and setting up unidirectional data flow. This means creating a reducer to manage your state, and using action creators to communicate with the Redux store.
You can initialize your object's default values in the reducer. This is returned by default from the Redux store.
Your reducer would listen for an UPDATE_PERSON action, which would carry the payload for the entire updated Person object. This would be stored in state, as below:
reducers/person.js
const UPDATE_PERSON = 'UPDATE_PERSON';
const initialState = {
name: "Tim",
age: 23
}
const personReducer(state = initialState, action) {
switch (action.type) {
case UPDATE_PERSON:
return {
...state,
name: action.payload.name,
age: action.payload.name
}
default:
return state;
}
}
Your action creator is a simple function with a type property and some kind of payload:
(presumably) actions/person.js
export const updatePerson(data) {
return {
type: UPDATE_PERSON,
payload: data
}
}
You then connect the Redux store to your component, and use the action creator to dispatch the action to the store:
import { connect } from 'react-redux';
import * as PersonActionCreators from '../actions/person';
class App extends Component {
changeName() {
this.props.updatePerson({name: "Jane", age: 22});
}
render() {
return (
<div>
<div>Your name is: {this.props.person.name}</div>
<div>Your age is: {this.props.person.age}</div>
<div><button onClick={this.changeName.bind(this)}>Change Name</button></div>
</div>
)
}
}
const mapStateToProps = (state) => ({
person: state.person
});
const mapDispatchToProps = {
updatePerson: PersonActionCreators.updatePerson
};
export default connect(mapStateToProps, mapDispatchToProps)(App);
The above code assumes you have a root reducer in the following format:
import { combineReducers } from 'redux';
import personReducer from './reducers/person';
const appReducer = combineReducers({
person: personReducer
})
const rootReducer = (state, action) => appReducer(state, action);
export default rootReducer;
You will need to create the store and connect your root reducer to it. Details on that can be found here.
The combineReducers function simply helps to construct the root reducer:
The combineReducers helper function turns an object whose values are
different reducing functions into a single reducing function you can
pass to createStore.
This is more boilerplate, but it is the established and most popular way of handling application state in React. It may seem like a lot to wrap your head around at first, but once you become familiar with reducers, action creators, and the connect function, it becomes very straightforward.
Redux uses a uni-directional data-flow, which means data streams downwards from the top-level components to child components. Stateful components are kept to a minimum; but where state is required, the connect function provides it. When a component needs to modify state, it does so through an action creator. The reducers listen to actions and update state accordingly.
See Dan Abramov's free egghead courses on this topic for an excellent introduction to Redux:
Getting started with
Redux
Building React Applications with Idiomatic
Redux

Answer 2

It's simple. The right way is using state for properties you want to display.
In this case, your code should be
setState({
name: this.person.name,
age: this.person.age
});
Why? Well, it's best practice, and using this.state is encouraged in docs. Attaching properties on the component (this), is generally usual for methods.
Also, consider the component methods shouldComponentUpdate(nextProps, nextState), componentWillUpdate(nextProps, nextState), componentDidUpdate(prevProps, prevState).
Obviously, if you need their arguments, you will not be able to retrieve the old/new properties you change, if them're not on state or props.