How to avoid common mistakes when you start writing Coffeescript
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Coffeescript Gotchas

These are some of the things that tripped me up when I started to write Coffeescript. They may be useful to new Coffeescript developers.

Coffeescript's is not Javascript's!

Don't use Coffeescript's for .. in for Objects, even though you would have done that in Javascript.

  • Use Coffeescript's for .. in for Arrays.
  • Use Coffeescript's for .. of for Objects.
for i,o in ['a', 'b', 'c']
  # i=0 o='a'
  # i=1 o='b'
  # i=2 o='c'

for k,v of {a: 10, b: 20, c: 30}
  # k='a' v=10
  # k='b' v=20
  # k='c' v=30

You could use this mnemonic to remember: An item lives in a list. Keys are properties of objects.

Or another way to remember is that it's the exact opposite way around to Javascript's traditional and ES6's for...of!

Accidentally returning a list comprehension

If the last statement in your function is a loop, in many cases Coffeescript will believe that you wanted to return the result of that loop (even though the loop is not assigned to any variable).

consumePies: (pies) ->
  for p in pies

That generates and returns a new array of the things p.consume() returned. There is memory and processing overhead to doing this, but it probably won't break your program, so you won't notice Coffeescript is doing it unless you check the code.

If you actually wanted to return nothing, and don't want to build an array of results for that loop, then put return on the last line.

consumePies: (pies) ->
  for p in pies

Alternatively, put @ on the last line to return this, which will present a fluent interface to the caller.

Using return to return an object literal at the end of a function

If you want to return an object with properties, this is NOT the right way:


getPosition = ->
    x: 10
    y: 20

This will simply return undefined, not the object! It processes return as a single statement. (Argh, it's like semicolon injection all over again!)

You can use return \ but that is ugly and easy to forget.

getPosition = ->
  return \              # Works but just NO!
    x: 10
    y: 20

I recommend you get into the habit of placing the object literal directly at the end, without a return:

getPosition = ->

  x: 10
  y: 20

Yes that is how Coffeescript does it! The empty line is optional but I find it clearer.

You cannot shadow outer variables (within a file)

If, inside a function, you assign a variable which was declared somewhere above the function, no local variable will be created, but the outer variable will be assigned.

This can trip you up if you aren't careful, and has been the subject of some debate.

How to avoid issues? When you introduce a new variable, search to see if it has been used anywhere else in the file (specifically in a parent or child scope of the current scope). If it is already used, then choose a unique name for your new variable (or rename the existing one).

Arguments may be passed without brackets - is that good?

For example you can do parseInt '5', 10 but a call without arguments must always use ()s, e.g. Math.random(). Sometimes I would forget, and type something like:

print "Hi mum"
print                        # Wrong - does not call print()!
print "Y U no blank line?"

or even dumber:

x = Math.cos angle
y = Math.sin angle
z = Math.random              # Wrong - z is now a function not a number!

The solution is to use print() and Math.random() for the no-argument calls. This looks inconsistent. Partly for that reason, I use parentheses on most of my function calls.

print("Hi mum")
print()                      # Now works fine
print("Y U no blank line?")

x = Math.cos(angle)
y = Math.sin(angle)
z = Math.random()            # and I didn't have to think about it!

Coffeescript is greedy when parsing parameters

Another reason to use brackets. Without brackets, there is ambiguity regarding where your parameters are going, requiring extra cognition for the developer. Sometimes arguments you intended for one function will actually be passed to the other. For example:

subtractNumbers 12, addNumbers 5,7         # Does that mean

subtractNumbers(12, addNumbers(5,7))       # this

subtractNumbers(12, addNumbers(5), 7)      # or this?

Since parameter parsing is greedy, not BODMAS:

Math.sin t/77 + Math.cos t/99         # CS, looks good


Math.sin(t / 77 + Math.cos(t / 99))   // JS, oh it wasn't good!

I find using brackets is clearer and avoids any ambiguity.

You will need brackets sooner or later anyway

When you have a complex expression, you will need brackets anyway. So your choice comes down to:

Math.sin(t/77) + Math.cos(t/99)       # Looks like Javascript

(Math.sin t/77) + (Math.cos t/99)     # Looks like Lisp

Perhaps you like your code to look like Lisp, in which case, go ahead! For me, since CS lives in the JS world, I like to keep the expressions looking the same.

When to skip brackets

There is one case when skipping the parentheses is fantastic, and that is for a "trailing callback" (when you pass a callback function as the last argument):

fs.readFile filename, (err,contents) ->
  console.log "Contents of #{filename} are:", contents

In this case if you had used brackets for the readFile call, you would need an extra line at the end with just the closing ). Heinous!

fs.readFile(filename, (err,contents) ->
  console.log "Contents of #{filename} are:", contents

Plus means something special if unevenly padded

a + ',' + b      =>      a + ',' + b;

a+','+b          =>      a + ',' + b;

a +','+ b        =>      a(+',' + b);   // oops!

You are forced to declare your callbacks in reverse

Coffeescript has no way to declare functions. You must assign them like any other variable. Therefore they cannot undergo the hoisting that you may be familiar with from Javascript.

This will work:

setTimeout( -> callMeLater() , 2000 )

callMeLater = -> alert("Done")

But this will not:

setTimeout(callMeLater, 2000)

callMeLater = -> alert("Done")

Why not? Because we try to access callMeLater before it was assigned!

This would have actually worked fine in Javascript, because declared functions get hoisted to the top of their scope.

Unfortunately in the case above, there won't even be an error: setTimeout will be called with undefined and just do nothing - fiddly to debug!

So you need to define the callback first (even though it happens last):

callMeLater = -> alert("Done")

setTimeout(callMeLater, 2000)

In other words, you must write your code backwards! This messes up the flow when reading (and writing) the code.

One way to avoid this: don't name your callbacks unless you have to, just define them inline.

setTimeout( -> alert("Done") , 2000 )

Another way is to follow the first example above, and make an extra anonymous callback function that calls your named function.

setTimeout(callMeLater, 2000)           # The problem version

setTimeout( -> callMeLater() , 2000 )   # The safe version (less efficient)

Another option is to defer initialisation:

init = ->
  setTimeout(callMeLater, 2000)

callMeLater = -> alert("Done")


Which is ok provided you: don't put things in the init function which the callback will need to access later, and if using the pattern a lot, be careful that the init = -> of an inner scope does not overwrite the init of an outer scope before the outer init has been called!

There is no ternary

In Javascript the ternary conditional expression is:

condition ? result_if_true : result_if_false

But in Coffeescript you must use the longer:

if condition then result_if_true else result_if_false

However Coffeescript does have some other shortcuts which are useful in specific situations (e.g. the existence operator ?).

(LiveScript introduces a nice Haskell-like | cond = result syntax.)

Tips (not gotchas)

Immediately calling an anonymous function

If you want to call a function immediately after defining it:

myObject = ( ->
  privateThing = 3
  publicThing: -> privateThing*2

You can avoid the brackets by using the do keyword:

myObject = do ->
  privateThing = 3
  publicThing: -> privateThing*2

Assistance for Vim users

I wrote a helpful Vim plugin that assists when learning Coffeescript. Each time you save the file, it shows what new changes were made to the compiled JS file, as an diff in the preview window. It can help when experimenting with Coffeescript, to glance and ensure the output is what you expected it to be.

One other small gotcha is accidentally editing the compiled JS file instead of the source CS file. (I occasionally did that, before sourcemaps, when I opened the JS file to inspect a line number reported in a runtime error.) To avoid making that mistake, I have another little vim script which makes the JS file un-editable if the "Generated by Coffeescript" message is detected at the top.

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