setTimeout中的变量表示它未定义，但在外部定义时 [英] Variable inside setTimeout says it is undefined, but when outside it is defined

问题描述

我上课了。我需要在超时内做一些http工作。我面临的问题是超时内的http变量一直说它是未定义的。

I have a class. I need to do some http work inside of a timeout. The problem I am faceing is the http variable inside the timeout keeps saying it is undefined.

export class MyClass {

    http:Http:

    constructor(private http:Http) {
        this.http = http;
    }

    sendFriendRequest(){

    this.http.post( ...//http variable is defined here
           setTimeout(function(){
               this.http.post(...  //http is not defined here
        }
   }
}

推荐答案

原因是setTimeout 中的回调函数处于不同的词汇环境。这就是为什么ES6 +函数可以使用 => 来定义。这样，函数中的代码与函数共享相同的范围。

The reason for this is that the callback function inside setTimeout is in a different lexical environment. This is why in ES6+ functions can be defined using =>. This is so that the code within a function shares the same scope as the function.

要解决此问题，您可以使用ES6 +语法，而不是函数（a，b，args）{...} 你将使用（a，b，args）=> {...} ：

To fix this, you can either use ES6+ syntax, where instead of function(a,b,args) {...} you would use (a,b,args) => {...}:

setTimeout( () => {
  this.http.post(...)
});

或使用ES5语法：

var root = this;

setTimeout(function(){
    root.http.post(...)
}

希望这会有所帮助！

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