list comprehension without parentheses in a function - coffeescript

Question

I have a function that needs to return the last property of an object that satisfies the condition:
types = {
a: 1,
b: 2,
c: 3
}
g = (s) -> v for k, v of types when k is s
console.log g 'b'
this code prints [ 2 ]
I expected just 2, and not an array. And indeed, this code does print what I expect:
console.log v for k, v of types when k is 'b'
What is wrong?
P.S. I know that instead of this function I can just access the object's property using [], but this is a contrived example.

Answer 1

If we rearrange the code then things should be clearer.
Your second piece of code:
console.log v for k, v of types when k is 'b'
is just another way of writing this:
for k, v of types when k is 'b'
console.log(v)
or even:
for k, v of types
if k is 'b'
console.log(v)
Since there is only one 'b' key, only one console.log call is made.
Your first piece of code:
g = (s) -> v for k, v of types when k is s
is the same as this:
g = (s) ->
a = (v for k, v of types when k is s)
a
The loop, v for k, v of types when k is s yields an array by definition so a will be an array (with only one element) and g will return an array.

Answer 2

console.log v for k, v of types when k is 'b' will call console.log(v) for every v when k satisfies the condition whereas your first code snipped will call console.log(g(b)). If there were two elements in types that satisfied the condition, the outputs would be:
[1, 2]
and
1
2
To make g output the first element that satisfies the condition, you could use return with early out or just take the first element of the results array.
g = (s) -> return v for k, v of types when k is s