function eachKey(obj, fn, target) {
  for (let key in obj) {
    if (obj.hasOwnProperty(key)) {
      fn.call(target, key, obj[key]);
    }
  }
}
Loop over each key/value pair in a hash.

@param {Object} obj The object
@param {Function} fn The function to call
@param {?Object} target The object to bind to the function