Should You Use Callbacks with AngularJS Promises?

Callback Inversion in AngularJS: An Anti-Pattern

In AngularJS, it's common practice to pass callback functions to services that return Promises. However, this approach can introduce an anti-pattern and hinders the benefits of Promises.

Anti-Pattern Example:

Consider the following code, where the getTokens service accepts a callback function:

...
getTokens: function(callbackFn) {
  $http.get('/api/tokens').then (function onFulfilled(response) {
    callbackFn(response.data);
  });
}
...

This approach has several disadvantages:

• Inversion of Control: Callback functions execute within the service's context, inverting the control flow and making it hard to follow the code.
• Broken Promise Chaining:Callbacks prevent Promises from being chained for handling multiple events or manipulating data in sequence.
• Complexity: The explicit handling of callbacks adds unnecessary complexity to the codebase.

Refactoring for Best Practice:

To resolve this anti-pattern, refactor the code as follows:

...
getTokens: function() {
  return $http.get('/api/tokens');
}
...

In the calling module, use the returned Promise to handle the result:

yourModule.getTokens()
  .then(function(response) {
    // handle it
  });

Benefits of Best Practice:

Adopting this best practice provides several advantages:

• Preserves Promise Chaining: Promises can now be chained to perform multiple operations sequentially, improving code readability and maintainability.
• Simplified Code: Removing the explicit handling of callbacks simplifies the codebase and eliminates potential errors.
• Increased Testability: Promises provide a clean interface for testing asynchronous operations, making it easier to verify code behavior.

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