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Using string enum as key in {[key]: Type} throws not an index signature #22892
Description
TypeScript Version: 2.9.0-dev.20180325
Search Terms: string enum index signature cast
Code
enum Fruits {
MANGO = 'MANGO',
BANANA = 'BANANA',
}
type StringDict = { [key: string]: string };
function map(dict: StringDict, transform: (key: string, value: string) => void) {
const result = {};
for (const key of Object.keys(dict)) {
result[key] = transform(key, dict[key]);
}
return result;
}
map(Fruits, (key, value) => {
value.toLowerCase();
});
map(Fruits as StringDict, (key, value) => {
value.toLowerCase();
});Expected behavior:
Both map calls succeed, because a string enum is essentially a {[key: string]: string}. I ought to be able to use it anywhere that needs something indexed by string (as long as the values are appropriate).
Actual behavior:
index.ts(14,5): error TS2345: Argument of type 'typeof Fruits' is not assignable to parameter of type 'StringDict'.
Index signature is missing in type 'typeof Fruits'.
index.ts(19,5): error TS2352: Type 'typeof Fruits' cannot be converted to type 'StringDict'.
Index signature is missing in type 'typeof Fruits'.
Activity
mhegazy commentedon Mar 26, 2018
StringDicthas a stronger contract thanFruits, sinceFruits["someRandomString"]is notstring; and thusFruitis not aStringDict.the oposite is true as well, since the type of
Fruite.Mangois a specific literal type"MANGO"that is not just anystring.For these two issues the type assertion
Fruits as StringDictfails.consider defining the function as
objector as generic function inKwhereKis the keys of the object passed, both would allow you to call it onFruite. e.g.:or
appsforartists commentedon Mar 26, 2018
That's presuming I control the definition of
map.I feel like I ought to be able to pass a string enum into a function that requires a
[string]: stringand it should work. I understand your point about type theory, but it fails practically here.Is the solution when I don't control map to do
map(myEnum as {} as {[key: string]: string}? Not only is that ugly, but it doesn't protect me in the case that someone changes the shape of the enum (e.g. adding a non-string value).mhegazy commentedon Mar 27, 2018
it is unsafe, since the index signature on the enum is really
string | undefined. but i suppose we can bend the rules here the same way we do for object literals... worth discussing.appsforartists commentedon Mar 27, 2018
Thanks.
The index signature on
{a: 1, b: 2}is also reallystring|undefined, so accepting a string enum's potential for anundefinedkey feels consistent with that.dardino commentedon Mar 27, 2018
Another case with standard enums:
mhegazy commentedon Mar 27, 2018
Gabriele (@dardino) i am afraid this is a different issue. all objects can be indexed with strings/numbers and result in an
anyif not defined. it should be reported as an error under--noImplicitAny.RyanCavanaugh commentedon Aug 6, 2018
This would be super problematic because we always allow expressions of the form
expr.propnameifexprhas a string index signature, so there would be effectively zero "typo protection" onpropname. We don't want to be in the situation where removing or renaming a string enum key doesn't cause new errors to appear, or where misspelling a string enum key doesn't cause an error.royling commentedon Feb 3, 2021
This can now be easily done with the new template literal types introduced in 4.1: