Flow can't see that my array is not empty? - flowtype

Question

Considering the following code :
const size = 500
const indexes = new Array(size).fill(0).map((_,idx) => idx)
let vals
if (!vals) {
vals = new Array(size).fill(0)
}
indexes.forEach(v => vals[v]++)
Note: I understand this code might look strange. forEaching an array (indexes) and use value as index for another one (vals) might not be usual. But it works and I just tried to make a simple case of a real use-case.
Flow returns this error :
indexes.forEach(v => vals[v]++)
^ access of computed property/element. Computed property/element cannot be accessed on possibly undefined value
indexes.forEach(v => vals[v]++)
^ uninitialized variable
(You can try it on flow.org/try)
to remove the error, I have to change the last line to verify vals on each iteration :
indexes.forEach(v => vals && vals[v] && vals[v]++)
The error vanish also if I replace vals declaration with something initialised immediately
let vals = new Array(size).fill(0)
(So using an array values as indexes to another array doesn't seem to be a problem)
Shouldn't flow be able to understand that vals is defined ?
Thanks for any advice.

Answer 1

Flow is possibly not able to infer the value to that level so you could do like this
const size = 500
let vals
if (!vals) {
vals = new Array(size).fill(0)
}
let newVals = vals
vals.map(v => newVals[v]++)

Answer 2

The problem here isn't because flow thinks the array is empty. It's because flow assumes the conditional initialization is in fact conditional, whereas you know that the if branch will always be executed.
Flow is looking at the types, not the values. (Even though some of the values can be known at compile time)
As far as Flow is concerned,
if (<boolean expression here>) {
vals = new Array(size).fill(0)
}
may or may not initialize vals.
The easiest solution is to remove the unnecessary if to make it clear that vals is always initialized:
const size = 500
const indexes = new Array(size).fill(0).map((_,idx) => idx)
let vals
//if (!vals) {
vals = new Array(size).fill(0)
//}
indexes.forEach(v => vals[v]++)
Try it here
Or even better:
const size = 500
const indexes = new Array(size).fill(0).map((_,idx) => idx)
const vals = new Array(size).fill(0)
indexes.forEach(v => vals[v]++)
Try it here