From an array of objects, how to return property `b` of the object that has the highest property `a`?

I need to get the value of the property b from the object with the highest value of the property a.

var myArr = [
  {
    a: 1,
    b: 15
  },
  {
    a: 2,
    b: 30
  }
];

I tried the following, but it just returns the highest value of a, rather than of b.

var res = Math.max.apply(Math,myArr.map(function(o){return o.a;});
var blah = getByValue(myArr);