Function to sum an object values using recursion

functionalObjSum.js

// This is super slow though: http://jsperf.com/summing-objects/2

var sumObj = function (object) {
  if (Object.keys(object).length) {
    var firstKey = Object.keys(object)[0];
    var clone = Object.assign({}, object);
    delete clone[firstKey];
    return parseInt(object[firstKey]) + sumObj(clone);
  }

  return 0;
};

Two considerations:

1. As long as you don't mutate external state, you're fine using a loop. Don't jump through hoops just to avoid a loop. This fn is recursive, so you're still technically handling the looping logic. Just because you're not using for doesn't mean you're not micromanaging the looping logic.
2. I'm baffled about why you're doing this with an object at all. This could be handled in a straightforward style with an array & reduce(). Use the right data structure for the job at hand. =)

Rule of thumb: If you write more code avoiding a loop than you'd write if you wrote the loop yourself, that's not functional style.

Using functional style should result in less code, not more. e.g.,

export default (numbers) => {
  const doubled = [];
  for(var i = 0; i < numbers.length; i++) {
    var newNumber = numbers[i] * 2
    doubled.push(newNumber)
  }
  return doubled;
};

vs declarative functional style:

export default (numbers) => numbers.map(n => n * 2);

Wow! Thank you very much, Eric!

I see now it could've been a simple reduce.

var sumObj = function (obj) {
  return Object.keys(obj).reduce(function (sum, next) {
    return sum + obj[next];
  }, 0);
};

Looks even neater with ES6:

const sumObj = obj => Object.keys(obj).reduce((sum, next) => sum + object[next], 0)