The Broken Promise: How jQuery's Flawed Implementation Undermines JavaScript Asynchronicity

For over a decade, JavaScript developers have embraced promises as salvation from callback hell—yet many implementations fundamentally misunderstand their purpose. As Domenic Denicola originally exposed, libraries like jQuery perpetrate a dangerous illusion of promise compatibility while violating core asynchronous programming principles. This isn't merely academic: it fractures interoperability and sabotages error handling in production systems.

The Anatomy of a True Promise

Promises aren't just callback aggregators. As defined in the Promises/A+ specification, they establish a direct correspondence between synchronous and asynchronous code by:

// Synchronous
function getUserData() {
  const data = fetchData(); // Blocking
  return process(data);
}

// Asynchronous equivalent
function getUserDataAsync() {
  return fetchDataAsync() // Returns promise
    .then(process); // Automatic value propagation
}

The critical feature lies in then's behavior:

"This function should return a new promise that is fulfilled when the given handler finishes [...] If the callback throws an error, the returned promise will be moved to failed state."

This enables four essential composition scenarios:

1. Value transformation: Handler returns value → New promise fulfills
2. Intentional failure: Handler throws exception → New promise rejects
3. Error recovery: Rejection handler returns value → New promise fulfills
4. Error propagation: Rejection handler throws → New promise rejects

jQuery's Fatal Flaws

Despite marketing promises, jQuery's implementation (including recent 2.x versions) fails scenarios 2-4. Its then method:

• Mutates existing promises instead of creating new ones
• Absorbs exceptions rather than propagating rejections
• Violates the Promises/A+ requirement that promises are immutable after settlement

Consider this dangerous example:

const jqPromise = $.get('/data');

jqPromise.then(() => {
  throw new Error('Explosion!');
});

// This should catch the error but doesn't in jQuery
jqPromise.catch(() => console.log('Never called!'));

As Denicola notes, this is equivalent to synchronous code where thrown exceptions magically vanish—an intolerable behavior for robust systems.

The Ripple Effect

jQuery's design inflicts concrete damage:

1. Interoperability collapse: Libraries expecting Promises/A+ compliance fail unpredictably with jQuery objects
2. Debugging nightmares: Swallowed exceptions create ghost failures
3. Architectural contamination: Teams build layered abstractions on flawed fundamentals

One jQuery core member admitted these flaws would persist indefinitely for backward compatibility, creating a permanent rift in the JavaScript ecosystem.

The Path to Sanity

Thankfully, Promises/A+ has evolved into a rigorous specification with vetted implementations:

• Q: Feature-rich with Node.js adapters and debugging tools
• RSVP.js: Minimalist and spec-compliant
• when.js: Adds advanced concurrency control

For legacy systems, assimilate jQuery promises immediately:

import Q from 'q';

const realPromise = Q.when($.get('/data')); // Now behaves correctly

Modern JavaScript has largely converged on Promises/A+ through native implementations and async/await syntax. Yet jQuery's stubborn divergence remains a cautionary tale: when foundational abstractions fracture, the entire ecosystem pays the maintenance debt. As Denicola presciently warned, misunderstanding promises isn't benign—it corrodes our ability to reason about asynchronous workflows at scale.