Tuesday 7 August 2018

Promises in node.js: Helper functions

Today we will have a look at some of the cool helper methods provided by the native promise framework. The first couple are just quick helpers to create promises in different states.

These methods quickly wrap values in promises of either resolved or rejected state. This will then execute the path that that promise would normally go down (i.e. .then() for resolved, .catch() for rejected).

In this example we use a made up httpGet function and pass in urls to get an array of returned data. The simplest way to think about Promise.all is that it takes an array of promises and returns a single promse that resolves when all of the promises in the array are complete.

This helper is similar to all except as the name suggests it just returns the first one that completes, this could potentially be a way of implementing timeouts on things, I find myself using Promise.all() much more frequently, but still it is good to understand!

Finally blocks are useful to help you close down resources no matter what happens.

Just a quick note to show that promises can have .then() put on them after they complete and the chain will still be executed. The reason for attaching this inside of another .then() was so that the eventloop would get a chance to fire before attaching the new one.

More promises coming soon!

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