combine.js

const combine = (...args) =>
  [].slice.call(args).reduce((next, value) => {
    // Create an object with duplicates combined
    const duplicates = Object.keys(value)
      .filter(key => next.hasOwnProperty(key))
      .map(key => {
        return {
          [key]: Object.assign(value[key], next[key])
        };
      })
      .reduce(
        (accumulator, current) => Object.assign(accumulator, current),
        {}
      );

    // Create a new object with everything combined together
    return Object.assign(next, value, duplicates);
  });