'this' in function inside prototype function [duplicate]

Answer 1

tl;dr

Use arrow functions. They are available since ECMAScript 6:

var sampleObject = function() {
  this.foo = 123;
}

sampleObject.prototype.getFoo = function() {
  var nested = () => { // Changed this line.
    return this.foo;
  }
  return nested();
}

var test = new sampleObject();

window.alert(test.getFoo());

Explanation

This is one of the main advantages of arrow functions. Your case is described in the section: No binding of this. The refference states:

Until arrow functions, every new function defined its own this value [...] An arrow function does not create its own this context, so this has the original meaning from the enclosing context.