Promises

The traditional way to handle an asynchronous operation is to use callback functions for the success or failure of the operation. If the operation succeeds, the success function is ­called; otherwise, the failure function is called. The following (abstract) example shows the idea of using a callback function:

function doAsyncCall(success,  failure) {
  // Do some API call
  if (SUCCEED)
    success(resp);
  else
    failure(err);
}
success(response) {
  // Do something with response
}
failure(error) {
  // Handle error
}
doAsyncCall(success, failure);

Nowadays, promises are a fundamental part of asynchronous programming in JavaScript. A promise is an object that represents the result of an asynchronous operation. The use of promises simplifies the code when you’re executing asynchronous calls. Promises are non-blocking. If you are using an older library for asynchronous operations that doesn’t support promises, the code becomes much more difficult...