React.Componentのメンバ変数は、再レンダー時に更新されるとは限らない！

import React from 'react'
import ReactDOM from 'react-dom'
import Counter from './counter'

class App extends React.Component {
  constructor(props) {
    super(props)
    this.state = {count: 0}
    this.increment = this.increment.bind(this)
  }

  increment() {
    this.setState(
      (state) => {return {count: state.count + 1}}
    )
  }

  render() {
    return (
      <Counter count={this.state.count} onClick={this.increment} />
    )
  }
}

let app = document.getElementById('app')
ReactDOM.render(<App />, app)

import React from 'react'

class Counter extends React.Component {
  constructor(props) {
    super(props)
    this.count = this.props.count
  }

  render() {
    // 以下の`this.count`は、新しいpropsを反映しない！
    return (
      <div>
        <p>{this.count}</p>
        <button onClick={this.props.onClick}>カウント</button>
      </div>
    )
  }
}

export default Counter

import React from 'react'

class Counter extends React.Component {
  constructor(props) {
    super(props)
  }

  render() {
    return (
      <div>
        <p>{this.props.count}</p>
        <button onClick={this.props.onClick}>カウント</button>
      </div>
    )
  }
}

export default Counter

class Counter extends React.Component {
  constructor(props) {
    super(props)
  }

  render() {
    let count = this._getCount()
    return (
      <div>
        <p>{count}</p>
        <button onClick={this.props.onClick}>カウント</button>
      </div>
    )
  }

  _getCount() {
    let count;
    // すごく複雑な処理
    return count
  }
}