js確保正確this的幾種寫法

1.直接用bind呼叫

this.method.bind(this)

2.建構函式中用bind定義

class Foo{
    constructor(){
        this.method = this.method.bind(this);
    }
}

3.用箭頭函式定義或呼叫

//定義
class Foo{
    constructor(){
        //定義1
        this.method1 = () => { this.xxx = b; }
    }
    //定義2
    method2: () => { this.xx = a; }
}

//呼叫
this.someThingAsync()
  .then(() => {
    this.method()
  })