What is the best way to stop a series of Ajax promises from continuing?

Managing multiple ajax requests that are dependent on each other can be tricky, especially when you need to stop the chain if one of the requests returns false.

Check out this sample code snippet below:

// Implementing a promise chain

return this.getBand(id)
            .then(this.getAlbum)
            .then(this.getSong);

// Ajax request for getBand
function getBand(id) {
  return Ember.$.ajax({
    data: { id: id },
    url: urls.bandUrl,
  }).then(function(result) {
    return result;
  });
};

// Ajax request for getAlbum
function getAlbum(result) {
  if (result.pass) {
    var bandName = result.name;
    return Ember.$.ajax({
      //...
    })
  } else {
    // How can we effectively stop the promise chain here?
  }
}

Answer №1

To signal an issue within the sequence, you can trigger a rejected Deferred:

function retrieveAlbum(result) {
  if (result.invalid) {
    // ...
  } else {
    return Ember.$.Deferred().reject('Previous result is invalid');
  }
}

Alternatively, you can modify retrieveBand() to directly verify result.invalid, preventing the execution of retrieveAlbum() unless it meets the criteria.

function retrieveBand(id) {
  return Ember.$.ajax({
    // ...
  }).then(function(result){
    return result.invalid ?
      result :
      Ember.$.Deferred().reject('No matching band found for ID ' + id);
  });
};

The chain will not halt completely, but only move to designated fail callbacks/filters specified in the .then() method as a secondary parameter or through .fail().

return this.retrieveBand(id)
    .then(this.retrieveAlbum)
    .then(this.retrieveSong)
    .fail(function (error) {
        // Inform user about the occurred error
    });

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