The Little Book on CoffeeScript

Loops and Comprehensions

Array iteration in JavaScript has a rather archaic syntax, reminiscent of an older language like C rather than a modern object-orientated one. The introduction of ES5 improved that situation somewhat, with the forEach() function, but that still requires a function call every iteration and is therefore much slower. Again, CoffeeScript comes to the rescue, with a beautiful syntax:

for name in ["Roger", "Roderick", "Brian"]
  alert "Release #{name}"

If you need the current iteration index, just pass an extra argument:

for name, i in ["Roger the pickpocket", "Roderick the robber"]
  alert "#{i} - Release #{name}"

You can also iterate on one line, using the postfix form:

release prisoner for prisoner in ["Roger", "Roderick", "Brian"]

As with Python comprehensions, you can filter them:

prisoners = ["Roger", "Roderick", "Brian"]
release prisoner for prisoner in prisoners when prisoner[0] is "R"

You can also use comprehensions for iterating over properties in objects. Instead of the in keyword, use of:

names = sam: seaborn, donna: moss
alert("#{first} #{last}") for first, last of names

The only low-level loop that CoffeeScript exposes is the while loop. This has similar behavior to the while loop in pure JavaScript, but has the added advantage that it returns an array of results (i.e. like the Array.prototype.map() function):

num = 6
minstrel = while num -= 1
  num + " Brave Sir Robin ran away"