Defer().promise vs. Promise: Which JavaScript Promise API is Safer?

Understanding the Distinction Between defer().promise and Promise in JavaScript

Promises in JavaScript provide an elegant way to handle asynchronous operations. Depending on the library you use, you may encounter two distinct APIs related to promises: defer().promise and Promise.

The Legacy Defer API

The defer() function in the legacy API creates a deferred object, which represents the promise state (resolved or rejected) and provides methods to manually control it. You can resolve the promise by calling the resolve() method, which accepts a value that becomes the result of the promise. The promise returned by defer().promise encapsulates this deferred object.

The Promise Constructor

The Promise constructor creates a promise directly, allowing you to specify the resolver and rejecter functions as arguments. These functions represent the actions that will fulfill or reject the promise respectively.

The Difference: Throw Safety

A key difference between these two APIs lies in throw safety. The Promise constructor is throw-safe, meaning that if an exception is thrown within the executor functions (resolver or rejecter), the promise will be rejected instead.

In contrast, the legacy defer API is not throw-safe. If an exception is thrown within the deferred object's methods (resolve() or reject()), it will be propagated synchronously, which can lead to unexpected errors.

Recommendation

Due to the improved throw safety and modern syntax, the Promise constructor is the recommended API for working with promises. It simplifies error handling and prevents common programmer errors.

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