Question

I know how to do a incrementing for loop in coffeescript such as:

Coffeescript:

for some in something

Generated Javascript:

for (_i = 0, _len = something.length; _i < _len; _i++)

How do I create a decrementing for loop similar to this in Coffeescript?

for (var i = something.length-1; i >= 0; i--)

Answer 1

EDIT: As of CoffeeScript 1.5 by -1 syntax is supported.

First, you should familiarize yourself with the by keyword, which lets you specify a step. Second, you have to understand that the CoffeeScript compiler takes a very naïve approach to loop endpoints (see issue 1187, which Blender linked to), which means that

for some in something by -1 # don't do this!!!

will result in an infinite loop—it starts at index 0, increments the index by -1, and then waits until the index hits something.length. Sigh.

So you need to use the range loop syntax instead, which lets you specify those endpoints yourself—but also means you have to grab the loop items yourself:

for i in [something.length - 1..0] by -1
  some = something[i]

Obviously that’s pretty messy. So you should strongly consider iterating over something.reverse() instead. Just remember that reverse() modifies the array that you call it on! If you want to preserve an array but iterate over it backwards, you should copy it:

for some in something.slice(0).reverse()