A CoffeeScript gotcha

20 Feb 2015

I like CoffeeScript, it is an improvement on plain JavaScript in many ways but it’s not without it’s own drawbacks and gotchas.

Implicit returns and _.forEach

So recently I spent some time staring at a piece of code that looked something like this:

_.foreach fruits, (fruit) ->
  if fruit.isApple()
    dontEatIt()
  else
    eatIt()

Here I’m using the excellent lodash library and it’s forEach method to iterate over a collection of fruits containing, lets say some oranges, apples and a few bananas.

So I found that the code ran the way that I expected it to as it processed oranges and bananas but as soon as I hit an apple it stopped processing any more fruits. It wasn’t until I read the docs for _.forEach (that after a fair bit of head scratching) that I worked out what was going wrong.

It just so happened that the dontEatIt method was returning false value whereas the eatIt method returns a truthy value. Add to this the fact that you can terminate a _.forEach loop by simply returning false from the callback and it all became clear, maybe I was being a bit slow.

It seems lodash is consistent with jQuery.each in this respect, though not with the new Array.forEach method introduced in ES5. Also, to add to the confusion, _.forEach does not stop iterating if you return one of JavaScript’s many other falsey values, so if dontEatIt() had simply returned undefined I would not seen this problem.

Would I choose to use CoffeeScript?

For some people it’s use of significant whitespace is a major red flag but I like using CoffeeScript from time to time. I took the trouble to learn the basics 3-4 years ago when it was pretty new and being pushed by the Rails community. Since then I’ve become less keen and haven’t set out to use it any projects where the choice is down to me.

There are a couple of reasons for this. Firstly, it adds another layer of complexity to the tool chain and adds another layer that the developer needs to understand. The important point here is that I still need to know JavaScript as well to understand what I’m writing and to debug any issues. And when a new developer joins the team, perhaps fairly new to JavaScript, is it fair to expect her or him to pick up both languages at once?

Secondly, ES6 addresses many of the same issues that CoffeeScript was designed to fix so in the future there is going to be very little need for it. So it seems as though its best years are behind it. I know the feeling.