How to use a custom function with an ArrayFormula - google-apps-script

Question

I want to write a function that can be used inside an ArrayFormula. My table is like this:
| A | B | C |
1| a | | |
2| b | | |
3| c | | |
First I wrote a simple function to return the input (so I know it works inside the ArrayFormula):
function retAddress(cell){
return cell;
}
On B1 I wrote =ArrayFormula(retAddress(address(row(B:B),column(A:A),4))) and apparently it worked as expected, it returned each address, like this:
| A | B | C |
1| a | A1| |
2| b | A2| |
3| c | A3| |
Now, on column C, I wanted to return the values of column A, so I wrote a function like this:
function retValue(cell){
var cellRang = SpreadsheetApp.getActive().getRange(cell);
return cellRang.getValue();
}
And on C1 I wrote =ArrayFormula(retValue(address(row(B:B),column(A:A),4))) but it gives me error Exception: Range not found (line 2)., which is the line with getRange(cell) method.
If I write the function without ArrayFormula like this:
On C1, =retValue(address(row(C1),column(A:A),4))
On C2, =retValue(address(row(C2),column(A:A),4))
On C3, =retValue(address(row(C3),column(A:A),4))
I get the expected result:
| A | B | C |
1| a | A1| a |
2| b | A2| b |
3| c | A3| c |
So, how to make it work in ArrayFormula?

Answer 1

Issue:
SpreadsheetApp.getActive().getRange(cell)
cell is array if you provide a array input. getRange method expects a single string as input.
Solution:
map the array to single value
References:
Custom Function#Optimization
Best practices
Snippet#1:
function retValue(cell){
if(cell.map) {
return cell.map(retValue);
} else {
var cellRang = SpreadsheetApp.getActive().getRange(cell);
return cellRang.getValue();
}
}
Snippet#2:
Note that in the previous snippet you're calling getValue() 1 time per each cell in the input array. This is extremely slow. Better way is to call it as a batch:
=retValues("A1:B4")
function retValues(cell){//modified
var cellRang = SpreadsheetApp.getActive().getRange(cell);
return cellRang.getValues();//modified
}
Note that:
Only 1 call to getValues() is made.
Formula returns a array without explicit use of =ARRAYFORMULA(). All custom formulas are by default, array formulas, but they need to be configured to return values as arrays in apps script.