Why is this returning NAN NAN NAN

var Maths = {
  numbers: [1, 2, 3],
  multiplier: 2,
  multiplied: function () {
      // within this function 'this' refers to the object
      // assigned to Maths. That's why you can get the length
      // of 'numbers': 
      //    alert(this.numbers.length);
      //    alert(this.toString()); // [object Object]
      
      // "capture" a reference to the object assigned to Maths
      var self = this;
      
      return this.numbers.map(function(n) {
          // However, within this scope, 'this' is the window object
          //     alert(this.toString()); // [window Object]
          // not the object assigned to Maths.  Since you need
          // a reference to the object assigned to Maths, you need 
          // to use the "captured" reference
         return n * self.multiplier;
      });
  }
};

Maths.multiplied();

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In case it's not obvious, since you are assigning a function to "multiplied", that function is owned/bound to the object being assigned to Maths.  Thus, within that function the context of 'this' refers to the Maths object.  By contrast, in the callback to map, that anonymous function is not bound/assigned to any (custom) object.  So the default context of 'this' is the window object.

In case you are interested:
http://ryanmorr.com/understanding-scope-and-context-in-javascript/