And I remember the quote from the "Rich Dad, Poor Dad" author: "I'm a bestselling author, not best writing author".
Anyway, I will try in November.
If it is something like "Introduction to JavaScript" I think it might almost write itself. One could even take the structure from some book that already exists.
I was surprised that JavaScript courses are not that popular (or so it seems), and the most popular I have seen so far are Python courses (in terms of general programming courses - not sure what the most successful courses are overall). I've seen a Python course with 40K viewers, at 99$ a pop.
Just do a quick Google search and you can find many anecdotes about how writing a book is not even close to a good way of spending your time if your goal is to make money. And that's for authors with an established reputation in the field.
It's not impossible but it's really really difficult.
Books are indeed a tough sell, but that's partly because if you publish with a traditional publisher you'll be lucky to get more $10k advance for a developer reference and $20k for a mainstream Windows/OS X book, and even luckier - like lottery-winner lucky - if that advance ever earns out and you see some royalties.
That's the high end. The low end is somewhere between zero and not much.
But if you can find an empty-ish niche and self-publish, your chances of a reasonable income are much higher. And if you can support that with live classes, sample code, a forum, and video content, I'd say the chances of making reasonable money are much better than in the crapshoot that is self-funded app development.
That's not even taking into account the fact that to produce such content the person has to be a (recognized especially if you don't have the marketing budget of large publishers) expert in a niche area. Such a person can presumably already make decent money as a consultant.
Being a consultant and a content producer is not mutually exclusive of course, but it's not quite the easy income I sense the original poster is looking for.
It's somewhat the same idea as an expert engineer inventing and patenting a machine design, then licensing it: they take a bunch of knowledge they would have sold once, and make it into something they can sell over and over.
"The Rich Dad, Poor Dad" targeted a huge generic market vs nich tech market for "Javascript" (very tough market). Your book will also be competing with Video tutors like:
http://www.lynda.com/search?q=javascript
For only $25/mo, you can have access to all the courses in their library.
How big is the market for the subject you want to teach about? What is already out there? What will differentiate your course from the other ones already out there, and how are you going to promote it?
I think I'll try it and there will be things to learn that are useful anyway, like how to make a good video presentation. That can come in handy even if I want to sell an app next.