Coffeescript gotchas (part 3): Switch statement and loops


Welcome to the third part of Coffeescript gotchas. If you haven’t yet read the first and second part of the series I strongly recommend to have a look. The main topic today is loops plus the switch statement. CoffeeScript offers a lot of enchantments and syntactic sugar to them.

Update: Table of contents
  1. CoffeeScript gotchas (part 1): Comments, strings, booleans, arrays and functions
  2. Coffeescript gotchas (part 2): Operators and aliases
  3. Coffeescript gotchas (part 3): Switch statement and loops
  4. Coffeescript gotchas (part 4): Scope and Objects

Switch statement

At last, an easier way to write switch statements. If your switch case contains a single line of code, you can write the entire thing in a single line using the then keyword (see second example).

name = "John"

# normal switch statement
switch name
  when "John"
    console.log "This guy rocks!"
  when "Jon"
    console.log "Still cool guy"
    console.log "Good lad."

# switch statement using the then keyword
switch name
  when "John" then console.log "This guy rocks!"
  when "Jon" then console.log "Still cool guy"
  else console.log "Good lad."

For Loops

For loops have many flavors and great potential in CoffeeScript. You can loop through arrays and objects and still keep your code clean and hassle-free. Just take a deep breath and try to absorb as many flavours as you can. They can all come very handy.


Iterating Arrays have never been easier…

arr = ["Ned Stark", "Jon Snow", "Arya Stark", "Jaime Lannister"]

# basic array iteration
for name in arr
    console.log name

# array iteration (with index)
for name, i in arr
    console.log "#{i}: #{name}"

# array filtering (with index)
for name, i in arr when name.indexOf "J" is 0
    console.log "#{i}: #{name}"

# array iteration for every second item
for name, i in arr by 2
    console.log "#{i}: #{name}"


Looping through an object’s key/value pairs is also very easy. You can also loop through just the direct key/value pairs (not the inherited ones) by using the own keyword (which will use the hasOwnProperty method under the hood).

ned =
  name : "Ned Stark"
  house : "Stark"
  words : "Winter is coming"

# print all properties of an object
for key, value of ned
  console.log "#{key} : #{value}"

# print all own properties of an object
for own key, value of ned
  console.log "#{key} : #{value}"

While Loops

The while loop has 3 syntax flavors to make your code more descriptive and your life easier. Try to use unless instead of while not, as it’s easier and preferable.

# Normal while
num = 10
while num
  console.log num

# while not
num = 10
while not (num is 0)
  console.log num

# exactly the same as the above example
num = 10
until num is 0
  console.log num

As you can see CoffeeScript is very handy when it comes to loops. Keep in mind and stay tuned for the fourth and final part of the Coffeescript gotchas series where we’ll have a look at the syntactic sugar that CoffeeScript provides on Scope and Objects.