Defer().promise vs Q.Promise: Which Promise API Should You Use?

Understanding the Distinction between defer().promise and Promise

Promises offer a paradigm shift in asynchronous programming, allowing for seamless handling of asynchronous operations. However, two distinct APIs coexist in the realm of promises: defer().promise and Q.Promise. This article explores their differences and the rationale behind their separate designs.

The Legacy of defer API

Defer() emerged as the initial promise handling mechanism, mirroring patterns seen in other languages and frameworks. It centers around the concept of deferring the resolution of a promise until later, allowing subsequent events to influence its outcome.

The Modern Promise Constructor

Q.Promise represents a more modern approach to promises, providing an alternative based on a completion source. This API enforces a stricter structure, where promises are constructed with explicit completion functions (resolve and reject).

The Need for Two APIs

While both APIs serve the same purpose, their design differences arise from the following:

Throw Safety

Promises are exceptionally throw safe. Any thrown exceptions within a promise chain automatically trigger rejection, ensuring consistent error handling. The defer().promise API, however, is not inherently throw safe, potentially leading to unhandled exceptions.

Use Case Comparison

Let's illustrate the key distinction with a simplified JSON parsing example:

// Using Defer()
get = function() {
  let d = Q.defer();
  if (cached) { d.resolve(parse(cached)); }
  else { myCallback('/foo', d.resolve); }
};

// Using Promise Constructor
get = function() {
  return new Promise((resolve, reject) => {
    if (cached) { resolve(parse(cached)); }
    else { myCallback('/foo', resolve); }
  });
};

In the defer() version, any exception during JSON parsing will result in a synchronous throw, requiring explicit error handling. In contrast, the promise constructor ensures throw safety, transforming exceptions into rejections within the promise chain.

Conclusion

The differences between defer().promise and Promise stem from their origins and the evolution of promise design. While defer() remains a widely used legacy, the promise constructor offers inherent throw safety, simplifying common programming errors. Understanding their respective strengths and weaknesses allows developers to make an informed choice based on their specific requirements.

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