TS allows strings to be string arrays? - arrays

Question

I've narrowed down an issue to the code below which is supposed to return a mapping from strings to string arrays. However, in my reducer I create a mapping from strings to strings, and TypeScript compiles the snippet without error:
const parseFeatures = (featSpecs: string[]): Record<string, string[]> =>
featSpecs.map(f => f.split("=", 2))
.reduce((acc, [k, v]) => {
// v is a string, so why does this compile?
return {...acc, [k]: v};
}, {});
console.log(parseFeatures(["foo=bar", "baz=qux,stuff,thing"]))
// correctly doesn't compile: Type 'string' is not assignable to type 'string[]'.
// const foo: Record<string, string[]> = {"foo": "bar"}
Running the snippet in TS playground gives this output:
{
"foo": "bar",
"baz": "qux,stuff,thing"
}
This is clearly not a mapping from strings to string arrays! How do I make TypeScript correctly type-check my reducer? I want this snippet to emit errors.

Answer 1

The issue here is that you are providing the empty literal to the reduce function. The return type is inferred from the type of that initial value and thus .reduce() returns {} which is assignable.
You can fix it by defining a type for the init value:
function parseFeatures(featSpecs: string[]): Record<string, string[]> {
const init: Record<string, string[]> = {}
return featSpecs
.map(f => f.split("=", 2))
.reduce((acc, [k, v]) => {
// this return vallue does not fit the init value type
return { ...acc, [k]: v };
}, init);
}
will give you the desired error:
Overload 2 of 3, '(callbackfn: (previousValue: Record<string,
string[]>, currentValue: string[], currentIndex: number, array:
string[][]) => Record<string, string[]>, initialValue: Record<string,
string[]>): Record<...>', gave the following error.
Argument of type '(acc: Record<string, string[]>, elem: string[]) => { [x: string]: string | string[]; }' is not assignable to parameter of type '(previousValue: Record<string, string[]>, currentValue:
string[], currentIndex: number, array: string[][]) => Record<string,
string[]>'.
Type '{ [x: string]: string | string[]; }' is not assignable to type 'Record<string, string[]>'.
'string' index signatures are incompatible.
Type 'string | string[]' is not assignable to type 'string[]'.
TS types may just describe a subset, so if you add a property to {}, TS will still consider it safe because you will not have access the the added properties afterwards. And then {} is just an empty record, so no problem there.