React source reading--constructor

react event source codes reading(v16.13.0)

article’s demo

import React from './react-dev/react';
import { render } from './react-dev/react-dom';
class App extends React.Component {
  constructor(props) {
    this.state = {count: 0};
    this.handleClick = this.handleClick.bind(this);

  handleClick(event) {
    console.log('event', event);
    this.setState((state) => {
        return {count: state.count + 1};

  render() {
    return [
        <button key="1" onClick={this.handleClick}>Update counter</button>,
        <span key="2">{this.state.count}</span>

render(<App />, document.getElementById('app'));

When we write the actual codes, we write super(props) every time when we write construtor, in fact, I don’t know what’s happen when we call it, according to the official documentation

When implementing the constructor for a React.Component subclass, you should call super(props) before any other statement. Otherwise, this.props will be undefined in the constructor, which can lead to bugs.

To see what’s going on from a source-code perspective, compile above demo with this command:

./node_modules/.bin/babel src/index.jsx

when super(props) is there

function App(propss) {
    var _this;

    _classCallCheck(this, App);
    // 注意这行
    _this =, propss);
    _this.state = {
      count: 0
    _this.handleClick = _this.handleClick.bind(_assertThisInitialized(_this));
    return _this;


function App(props) {
    var _this;

    _classCallCheck(this, App);

    // super(props);
    _this.state = {
        count: 0
    _this.handleClick = _this.handleClick.bind(_assertThisInitialized(_this));
    return _possibleConstructorReturn(_this);

See what I mean? super(props) is to define this, if not there, then this has no value, and all the rest of the codes will report an error. In fact, I think above official statement should be more rigid: you must call the super method when writing the constructor 🐶.

Also what’s props, it binds the props passed from parent.

In addition, when you call super, if your editor has a code hint, you can see that super can pass in props, context, updater three variables, but the general use is to use the first one, context is also rarely used here!