Why Do Chained Promises Sometimes Ignore Rejections?

Chained Promises Not Passing on Rejection

Despite their intended dependency on a previous promise, chained promises may fail to propagate errors. This behavior can be puzzling, but is rooted in the nature of Promises/A.

Misconception: Chain Dependency

The misconception is that each then implies dependency on the preceding promise's successful resolution. However, this is not the case.

Promise Chaining

In promise chaining, the result of the callback function in a then becomes the resolution of the subsequent promise. Therefore:

• If the callback returns a promise, the new promise is assimilated into the chain.
• If the callback does not return a promise or returns undefined, its value is passed directly to the next promise's callback.

Handling Rejection

Errors thrown or rejected promises in a then callback trigger the next promise's catch or error callback. However, if the error is handled within the callback, the chain proceeds as if the promise was successfully resolved.

Example:

Consider the following code:

let promise1 = Promise.resolve(42);
let promise2 = promise1.then(
  (value) => value * 2,
  (err) => {
    console.error('Error in promise1:', err);
    // Handle the error (e.g., return a new promise)
  }
);
promise2.then((value) => console.log('Value from promise2:', value));

In this example, if promise1 is rejected, its error handler logs the error and then returns a default value. This allows promise2 to proceed as if it had received a successful response from promise1.

Application to Database Updates

In your database update example, the error in db.query will be handled in its then callback and a default value will be provided. This allows the subsequent then callbacks to proceed and still perform the database updates, even though db.query failed.

Best Practices

To ensure consistent error handling, consider the following best practices:

• Use explicit error handling with catch or try-catch to handle rejections consistently.
• Re-throw or return rejected promises from error handlers to avoid propagating errors down the chain and ensure proper handling.
• Use Promise.all() to aggregate errors from multiple promises and handle them in a single place.

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