Question

From the selected answer in this SO-question this very ingenious function creates an Array with a range from 1 to i:

function range1(i){return i?range1(i-1).concat(i):[]}

It works perfect. Call me stupid, but I just can’t get my head around how it works. Let’s say we have range1(5). Now entering the function, we have i, so it returns itself with parameter i-1 (4) and concats i (5) to it. But here I’m stuck: how does range1 know it has to do with an Array? I’d say after the first run the return value (as long as we have i, so i!==0) would be a Number. And Number has no concat method. Can someone explain this? What am I missing?

Answer 1

Now entering the function, we have i,
so it returns itself with parameter
i-1 (4) and concats i (5) to it.

No, it doesn’t return itself. What it does is call itself, which is the recursion, then it returns the result of that call with the last element concatenated.

So, range1(5) will call range1(4), which will call range1(3), and so on. When it reaches zero it will stop making calls and just return an empty array.

range1(0) returns [], so range1(1) returns [].concat(1) which is [1], then range1(2) returns [1].concat(2) which is [1,2], and so on. When we return to range1(5)
it returns [1,2,3,4].concat(5) which is [1,2,3,4,5].

Note: This function works well to create small arrays, but if you need a large array it will be a lot faster to just create the array and fill it using a regular loop.