JavaScript with Promises

Chapter 5. Error Handling

One of the biggest benefits of using promises is the way they allow you to handle errors. Async error handling with callbacks can quickly muddy a codebase with boilerplate checks in every function. Fortunately, promises allow you to replace those repetitive checks with one handler for a series of functions.

The error handling API for promises is essentially one function named catch. However, there are some extra things to know when using this function. For instance, it allows you to simulate an asynchronous try/catch/finally sequence. And it’s easy to unintentionally swallow errors by forgetting to rethrow them inside a catch callback.

This chapter guides you through error handling in practice so you can write robust code. It includes examples using the standard Promise API as well as options the Bluebird promise library offers.

Rejecting Promises

Basic error handling with promises was introduced in Chapter 2 using the catch method. You saw how a rejected promise invokes callbacks registered with catch (repeated in Example 5-1.)

Example 5-1. Explicitly rejecting a promise

var rejectedPromise = new Promise(function (resolve, reject) {
    reject(new Error('Arghhhh!'));     // Explicit rejection
});

rejectedPromise.catch(function (err) {
    console.log('Rejected');
    console.log(err);
});

// Console output:
// Rejected
// [Error object] { message: 'Arghhhh!' ... }

The rejectedPromise was explicitly rejected inside the callback given to the Promise constructor. As ...