interview.js solution with modern schtuff

gistfile1.txt

function delegate (childIn) {
  var child = childIn;
  
  return function me(){
    var that = this,
        oper = Object.getOwnPropertyNames(that).filter(function (attrib) {
      return(typeof that[child][attrib] === 'function' && me === that[attrib]);
    })[0];

    return that[child][oper].apply(this[child], arguments);
  };
}
 
// usage
var obj = {
 
  math: {
    x: 2,
 
    add: function (y) {
      return this.x + y;
    },
 
    multiply: function (y) {
      return this.x * y;
    }
  },
 
  add: delegate('math'),
 
  multiply: delegate('math')
 
};

// test cases
console.assert( obj.add(1) === 3 );
console.assert( obj.multiply(3) === 6 );