Swap key with value JSON

Question

I have an extremely large JSON object structured like this:

{A : 1, B : 2, C : 3, D : 4}

I need a function that can swap the values with

Answer 1

As a complement of @joslarson and @jPO answers:
Without ES6 needed, you can use Object.keys Array.reduce and the Comma Operator:

Object.keys(foo).reduce((obj, key) => (obj[foo[key]] = key, obj), {});

Some may find it ugly, but it's "kinda" quicker as the reduce doesn't spread all the properties of the obj on each loop.