Test Your Promises

Test-Driving JavaScript Applications — by Venkat Subramaniam (28 / 108)

👈 Client-Side Callbacks | TOC | Wrapping Up 👉

Now let’s explore testing for promises in code running within Node.js. The same techniques work for client-side code that use promises as well. We’ll start with a short introduction to promises and then dive into testing code that uses promises.

Before digging into testing promises, let’s review how promises works — I promise to keep it short.

Traditionally asynchronous functions have used callbacks. But callbacks have a few drawbacks. Callbacks have to be registered before the asynchronous functions generate any response. For example, when using an instance of XMLHttpRequest, calling the send function before registering a callback with the onreadystatechange property may result in some events being missed. Also, it’s hard to compose callbacks — the code become overly verbose and cumbersome when a callback in turn has to invoke other asynchronous functions and register other callbacks.

Promises don’t have those issues and that’s one of the reasons they’re gaining in popularity. Functions that use promises immediately return a Promise object in response to calls. They then respond to the caller through the Promise object.

Promises more clearly express the intent of asynchrony than callbacks do. Here’s a figure that shows the propagation of promises through a series of function calls.

Code that uses promises may form a chain of then and catch calls. Either the top path or the bottom path in the chain is taken, depending on whether a promise is resolved or rejected. If the Promise is resolved, then the resolved part of the then function will transform the resolved value and pass it on, as a Promise, to the next receiver in the chain. On the other hand, if the Promise is rejected, then a series of catch functions transform the rejected reason through the call chain. This chain may also optionally include the rejected part of the then function.

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