code: function set(key, val, cb) {
    return type(key) != 'object'
      ? localForage.setItem(key, val).then(wrap(cb), cb)
      : Promise.all(Object.keys(key).map(setSubkey)).then(wrap(val), val);
  
    function setSubkey(subkey, next) {
      return key[subkey] === null
        ? del(subkey, next)
        : set(subkey, key[subkey], next);
    }
  }
docs: Set `val` to `key`.
  
  @param {Array|Mixed} key
  @param {Mixed} val
  @param {Function} cb