Recursive descent visitor pattern in JavaScript

visitor.js

'use strict'

const json = { 
  "a": ["A", "A", {"b": "B"} ],
  "c": { "d": "D", "e": ["E", "E"]},
  "f": "F"
}
function isPrimitive(obj) {
  if(null === obj) return true;
  if(undefined === obj) return true;
  if(['string', 'number', 'boolean'].some(type => type === typeof true)) return true;
  return false;
}

function visit(obj, visitor) {
  for(let key of Object.keys(obj)) {
    if(isPrimitive(obj[key])) {
      visitor(key, obj[key]);
    } else if(Array.isArray(obj[key])) {
      for(let item of obj[key]) {
        visit(item, visitor);
      }
    } else {
      visit(obj[key], visitor);
    }
  }
}

const out = [];

visit(json, (k, v) => out.push(v));

out;

As function or inline (7x slower, so performance VS readability choice):

function isPrimitive(test) {
    return (test !== Object(test));
};

Clever, @emirpasic.