RxJS: Avoiding Unbound Methods

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I often see code that looks a little like this:

Which seems fine.

Well, it is fine — as long as the extractSomeProperty and handleError methods do not depend upon the this context in their implementations.

Why might this be a bad thing?

When unbound methods are passed to RxJS, they will be invoked with an unexpected context for this. If the method implementations don’t use this, they will behave as you would expect.

However, there are a number of reasons why, as a general rule, you might want to avoid passing unbound methods:

• If you are in the habit of passing unbound methods, it’s a near certainty that you will, eventually, introduce a bug by passing, as unbound, a method that uses this in its implementation. Maybe you’ll have a test that picks it up; maybe you won’t.
• Passing unbound methods is something that’s often done with error handlers. And the unit testing of code paths for errors is often neglected.
• The callbacks passed to…