javascript - Variable inside setTimeout says it is undefined, but when outside it is defined

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
        }

   }
}

Answer

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.

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(...)

});

or with ES5 syntax:

var root = this;

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

Hope this helps!