[Loriel]

for (a: b) { ... } means that b should be an array, and the {} block is going to be executed once for every element in b, with a refering to each element in turn.

If b was an array of two sub-arrays, then the {} block would be executed twice, once with a as the first subarray and once with a as the second subarray.

: is magical as part of the for thing here, the : in ? : is something different: (a ? b : c) is an expression that is going to return b if a is true and c if a is not true. It tends to be a good idea not to use ? :.

Hope this helps.