Error by adding an value to an array, but I don't know how to solve it!

[nkamp]

I have todo some maintenance to a Vue project. From the backend records are send to the front-end and one field is picked to put the unique values in an Array to fill a dropdown with it.
`
...
const alleOvertredingen = ref<Array>([]);

const registrators = computed(() =>
unique(
alleOvertredingen.value .map((overtreding) => overtreding.handhavingDoor)
.concat(medewerker.value?.organisatie.naam)
).sort((a, b) => a.localeCompare(b))
);
...
`
The error which is returned:

Overload 1 of 2, '(...items: ConcatArray[]): string[]', gave the following error.
Argument of type 'string | undefined' is not assignable to parameter of type 'ConcatArray'.
Type 'undefined' is not assignable to type 'ConcatArray'.
Overload 2 of 2, '(...items: (string | ConcatArray)[]): string[]', gave the following error.
Argument of type 'string | undefined' is not assignable to parameter of type 'string | ConcatArray'.
Type 'undefined' is not assignable to type 'string | ConcatArray'.
549 | alleOvertredingen.value
550 | .map((overtreding) => overtreding.handhavingDoor)
551 | .concat(medewerker.value?.organisatie.naam)

If I replace medewerker.value?.organisatie.naam by a String like "Some value", than this value is added without error, so the idea is working in my opinion.

Or if I define a variable with let and in a watch() I add console.log, than the value is shown:
`
let medewerkerOrg = medewerker.value?.organisatie.naam;

watch(
console.log("random tekst: " + medewerkerOrg); // value is shown
...
`
If I define a 'const' instead of let, than there is still an issue, 'medewerkerOrg undefined'.

[chenxch]

This seems to have nothing to do with vue, but a problem you encountered using typescript.