This is as far as I was able to reduce the problem to:
interface Word {
value: string | string[];
}
const w: Word = {
value: [] as string[]
// also tried:
// value: []
// value: ['test']
}
w.value.push('')
The last line errors in my IDE because "push" does not exist for the type "string | string[]" (ts2339). It also wouldn't compile!
Is it a bug? Is it feature? How to help this?
[] as string[] doesn't really do much in your code, since it's already clear from the usage that this array would be a string array, and you then assign the object to a Word. You've defined Word so that value can be either a string or a string array, and while you might have started off with an array, the type you defined says you might change it to a string on a different line of code. Since the value might not be an array, typescript won't let you push to it unless you first verify that it's an array.
For example:
if (Array.isArray(w.value)) {
w.value.push('');
} else {
w.value = [w.value, ''];
}
If you want w to be a narrower type, one that's similar to Word but guarantees that the value is always an array, then do this:
const w: Word & { value: string[] } = {
value: []
}
string[]to avoid things like this.was being of typeWord, the compiler giveswthat type. Even though you initializew.valuetostring[], the compiler does not narrowwto{value: string[]}upon assignment. See ms/TS#33205. Personally I'd just not annotatewat all and let the compiler infer it as{value: string[]}to begin with.