javascript - Accessing this in a forEach loop results in undefined

Question

I'm iterating through an array using forEach in one of my Class's methods. I need access to the instance of the class inside the forEach but this is undefined.

var aGlobalVar = {};

(function () {

    "use strict";

  aGlobalVar.thing = function() {
      this.value = "thing";   
  }

  aGlobalVar.thing.prototype.amethod = function() {
      data.forEach(function(d) {
      console.log(d);
      console.log(this.value);
  });
}
})();

var rr = new aGlobalVar.thing();
rr.amethod(); 

I have a fiddle I'm working on here: http://jsfiddle.net/NhdDS/1/ .

Answer 1

In strict mode if you call a function not through a property reference and without specifying what this should be, it's undefined.

forEach (spec | MDN) allows you to say what this should be, it's the (optional) second argument you pass it:

aGlobalVar.thing.prototype.amethod = function() {
  data.forEach(function(d) {
    console.log(d);
    console.log(this.value);
  }, this);
  // ^^^^
}

Alternately, arrow functions were added to JavaScript in 2015. Since arrows close over this, we could use one for this:

aGlobalVar.thing.prototype.amethod = function() {
  data.forEach(d => {
    console.log(d);
    console.log(this.value);
  });
}