你还可以使用:

<button onClick={()=>this.delta()}>+</button>

Or:

<button onClick={event=>this.delta(event)}>+</button>

如果你正在传递一些参数..

你必须用这个关键字绑定新事件，正如我下面提到的…

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

        this.state = {
            count : 1
        };

        this.delta = this.delta.bind(this);
    }

    delta() {
        this.setState({
            count : this.state.count++
        });
    }

    render() {
        return (
            <div>
                <h1>{this.state.count}</h1>
                <button onClick={this.delta}>+</button>
            </div>
        );
      }
    }

你必须用'this'(默认对象)绑定你的方法。 不管你的函数是什么只要把它绑定到构造函数中。

constructor(props) {
    super(props);
    this.state = { checked:false };

    this.handleChecked = this.handleChecked.bind(this);
}

handleChecked(){
    this.setState({
        checked: !(this.state.checked)
    })
}

render(){
    var msg;

    if(this.state.checked){
        msg = 'checked'
    }
    else{
        msg = 'not checked'
    }

    return (
        <div>               
            <input type='checkbox' defaultChecked = {this.state.checked} onChange = {this.handleChecked} />
            <h3>This is {msg}</h3>
        </div>
    );

只需要更改bind语句 = > this.delta = this.delta.bind(this);

如果使用内部axios，则在then中使用箭头(=>)

Axios.get ('abc.com').then((response) => {});

虽然这个问题已经有了答案，但我还是想分享一下我的答案，希望对大家有所帮助:

/* 
 * The root cause is method doesn't in the App's context 
 * so that it can't access other attributes of "this".
 * Below are few ways to define App's method property
 */
class App extends React.Component {
  constructor() {
     this.sayHi = 'hello';
     // create method inside constructor, context = this
     this.method = ()=> {  console.log(this.sayHi) };

     // bind method1 in constructor into context 'this'
     this.method1 = this.method.bind(this)
  }

  // method1 was defined here
  method1() {
      console.log(this.sayHi);
  }

  // create method property by arrow function. I recommend this.
  method2 = () => {
      console.log(this.sayHi);
  }
   render() {
   //....
   }
}