React component initialize state from props

问题

In React, are there any real differences between these two implementations? Some friends tell me that the FirstComponent is the pattern, but I don't see why. The SecondComponent seems simpler because the render is called only once.

First:

import React, { PropTypes } from 'react'

class FirstComponent extends React.Component {

  state = {
    description: ''
  }

  componentDidMount() {
    const { description} = this.props;
    this.setState({ description });
  }

  render () {
    const {state: { description }} = this;    
    return (
      <input type="text" value={description} /> 
    );
  }
}

export default FirstComponent;

Second:

import React, { PropTypes } from 'react'

class SecondComponent extends React.Component {

  state = {
    description: ''
  }

  constructor (props) => {
    const { description } = props;
    this.state = {description};
  }

  render () {
    const {state: { description }} = this;    
    return (
      <input type="text" value={description} />   
    );
  }
}

export default SecondComponent;

Update: I changed setState() to this.state = {} (thanks joews), However, I still don't see the difference. Is one better than other?

回答1:

It should be noted that it is an anti-pattern to copy properties that never change to the state (just access .props directly in that case). If you have a state variable that will change eventually but starts with a value from .props, you don't even need a constructor call - these local variables are initialized after a call to the parent's constructor:

class FirstComponent extends React.Component {
  state = {
    x: this.props.initialX,
    // You can even call functions and class methods:
    y: this.someMethod(this.props.initialY),
  };
}

This is a shorthand equivalent to the answer from @joews below. It seems to only work on more recent versions of es6 transpilers, I have had issues with it on some webpack setups. If this doesn't work for you, you can try adding the babel plugin babel-plugin-transform-class-properties, or you can use the non-shorthand version by @joews below.

回答2:

You don't need to call setState in a Component's constructor - it's idiomatic to set this.state directly:

class FirstComponent extends React.Component {

  constructor(props) {
    super(props);

    this.state = {
      x: props.initialX
    };
  }
  // ...
}

See React docs - Adding Local State to a Class.

There is no advantage to the first method you describe. It will result in a second update immediately before mounting the component for the first time.

回答3:

Update for React 16.3 alpha introduced static getDerivedStateFromProps(nextProps, prevState) (docs) as a replacement for componentWillReceiveProps.

getDerivedStateFromProps is invoked after a component is instantiated as well as when it receives new props. It should return an object to update state, or null to indicate that the new props do not require any state updates.

Note that if a parent component causes your component to re-render, this method will be called even if props have not changed. You may want to compare new and previous values if you only want to handle changes.

https://reactjs.org/docs/react-component.html#static-getderivedstatefromprops

It is static, therefore it does not have direct access to this (however it does have access to prevState, which could store things normally attached to this e.g. refs)

edited to reflect @nerfologist's correction in comments

回答4:

You could use the short form like below if you want to add all props to state and retain the same names.

constructor(props) {
    super(props);
    this.state = {
       ...props
    }
    //...
}

回答5:

If you directly init state from props, it will shows warning in React 16.5 (5th September 2018)

回答6:

you could use key value to reset state when need, pass props to state it's not a good practice , because you have uncontrolled and controlled component in one place. Data should be in one place handled read this https://reactjs.org/blog/2018/06/07/you-probably-dont-need-derived-state.html#recommendation-fully-uncontrolled-component-with-a-key

回答7:

set the state data inside constructor like this

constructor(props) {
    super(props);
    this.state = {
      productdatail: this.props.productdetailProps
    };
  }

it will not going to work if u set in side componentDidMount() method through props.

回答8:

You can use componentWillReceiveProps.

constructor(props) {
    super(props);
    this.state = {
      productdatail: ''
    };
  }

    componentWillReceiveProps(nextProps){
        this.setState({ productdatail: nextProps.productdetailProps })
    }

来源：https://stackoverflow.com/questions/40063468/react-component-initialize-state-from-props

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