this.state vs state in React

问题:

I'm working in a new codebase. Normally, I would set up state like this in a React component:

class App extends React.Component {
    constructor() {
        super();
        this.state={
            foo: 'bar'
        }
     }
    ....

In this new codebase, I'm seeing a lot of this:

class App extends React.Component {
    state={
        foo: 'bar'
     }
    ....

Is there an advantage to doing it this way? They seem to only do it when state doesn't need to be altered. I always thought of state as being something React handled. Is this an ok thing to do?

回答1:

The end result of both approaches is the same. Both approaches are just setting the initial state of the component. It's worth noting that class properties are a stage 3 proposal, so all development environments may not be able to use them.

I personally like to use the class field variant if nothing else is done in the constructor, as it is less code to write, and you have no super call to worry about.

Example

class Component1 extends React.Component {
  state = { value: this.props.initialValue };

  render() {
    return <div> {this.state.value} </div>
  }
}

class Component2 extends React.Component {
  constructor(props) {
    super(props);
    this.state = { value: props.initialValue };
  }

  render() {
    return <div> {this.state.value} </div>
  }
}

function App() {
  return (
    <div>
      <Component1 initialValue={1} />
      <Component2 initialValue={2} />
    </div>
  );
}

回答2:

Actually both of them bind to this pointer. the this that made in constructor of class.

Totally you can access to local state by this.state but in first style you can pass props to constructor by super and then use it in state declaration, just like below:

class App extends React.Component {
    constructor(props) {
        super(props);
        this.state={
            foo: 'bar',
            jaz: props.someParentState,
        }
     }
....

Awesome, you can access to props in constructor, isn't pretty? I definitely use this style for local state declaration.

Hope this helps you.