interview.js

function delegate (child) {
  // make it happen
  return function inner () {
    var self = this;

    var func = Object.getOwnPropertyNames(this).filter(function (prop) {
      return inner === self[prop];
    }).pop();

    return this[child][func].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 );