Javascript reduce() on Object

Question

One option would be to reduce the keys():

var o = { 
    a: {value:1}, 
    b: {value:2}, 
    c: {value:3} 
};

Object.keys(o).reduce(function (previous, key) {
    return previous + o[key].value;
}, 0);

With this, you'll want to specify an initial value or the 1st round will be 'a' + 2.

If you want the result as an Object ({ value: ... }), you'll have to initialize and return the object each time:

Object.keys(o).reduce(function (previous, key) {
    previous.value += o[key].value;
    return previous;
}, { value: 0 });

What you actually want in this case are the Object.values. Here is a concise ES6 implementation with that in mind:

const add = {
  a: {value:1},
  b: {value:2},
  c: {value:3}
}

const total = Object.values(add).reduce((t, {value}) => t + value, 0)

console.log(total) // 6

or simply:

const add = {
  a: 1,
  b: 2,
  c: 3
}

const total = Object.values(add).reduce((t, n) => t + n)

console.log(total) // 6

ES6 implementation: Object.entries()

const o = {
  a: {value: 1},
  b: {value: 2},
  c: {value: 3}
};

const total = Object.entries(o).reduce(function (total, pair) {
  const [key, value] = pair;
  return total + value.value;
}, 0);

First of all, you don't quite get what's reduce's previous value is.

In you pseudo code you have return previous.value + current.value, therefore the previous value will be a number on the next call, not an object.

Second, reduce is an Array method, not an Object's one, and you can't rely on the order when you're iterating the properties of an object (see: https://developer.mozilla.org/en-US/docs/JavaScript/Reference/Statements/for...in, this is applied to Object.keys too); so I'm not sure if applying reduce over an object makes sense.

However, if the order is not important, you can have:

Object.keys(obj).reduce(function(sum, key) {
    return sum + obj[key].value;
}, 0);

Or you can just map the object's value:

Object.keys(obj).map(function(key) { return this[key].value }, obj).reduce(function (previous, current) {
    return previous + current;
});

P.S. in ES6 with the fat arrow function's syntax (already in Firefox Nightly), you could shrink a bit:

Object.keys(obj).map(key => obj[key].value).reduce((previous, current) => previous + current);

An object can be turned into an array with: Object.entries(), Object.keys(), Object.values(), and then be reduced as array. But you can also reduce an object without creating the intermediate array.

I've created a little helper library odict for working with objects.

npm install --save odict

It has reduce function that works very much like Array.prototype.reduce():

export const reduce = (dict, reducer, accumulator) => {
  for (const key in dict)
    accumulator = reducer(accumulator, dict[key], key, dict);
  return accumulator;
};

You could also assign it to:

Object.reduce = reduce;

as this method is very useful!

So the answer to your question would be:

const result = Object.reduce(
  {
    a: {value:1},
    b: {value:2},
    c: {value:3},
  },
  (accumulator, current) => (accumulator.value += current.value, accumulator), // reducer function must return accumulator
  {value: 0} // initial accumulator value
);