js
// define
function asyncFunction (id, callback) {
  const result = Client.fetch(id);
  callback(result);
}
// usage
asyncFunction(1, (result) => {
  // do something with your result
})

js
// define
function asyncFunction (id) {
  return new Promise((resolve, reject) => {
    const result = Client.fetch(id);
    if (result) {
      return resolve(result);
    }
    reject(new Error('Can\'t get result!'));
  });
}
// usage
asyncFunction(1)
  .then( (result) => {
    // do something with your result
  })
  .catch((error) => {
    console.error(error);
  });