Expose API with Promises and Optional Callback Feature

The API pattern below allows users to use it as a promise or as a callback-based API (if an optional callback argument is passed in):

function asyncFn(arg1, arg2, callback) {
  return new Promise(function(resolve, reject) {
    // When error occurs
    if(someError) {
      const err = new Error('Invalid input')
      if(callback) {
        callback(err)
      }
      return reject(err)
    }
    
    // No error
    if(callback) {
      callback(null, result); 
    }
    resolve(result);
  })
}

Some notes:

• The async API checks presence of callback function before calling it
• A promise is always created and completed regardless if users opt for the callback option.

Posted on: Mon, 04 Mar 2019 09:11:09 GMT

Tags:

javascript

design pattern