Closed
Description
TypeScript Version: reproducible in at least 3.3.3333 and above
Search Terms: Array map generic extra property
Code
type Token = {
value: string
}
const things = ['one', 'two'];
// using .map generic — doesn't fail typechecking
function getTokens1() {
return things.map<Token>(thing => ({
value: thing,
// should say `somethingElse` does not exist on type `Token`
somethingElse: true
}));
}
// using return type — doesn't fail typechecking
function getTokens2(): Token[] {
const tokens = things.map(thing => ({
value: thing,
// should say `somethingElse` does not exist on type `Token`
somethingElse: true
}));
// type of tokens is reported as `{ value: string; somethingElse: boolean; }[]`
// so how could it be compatible with `Token[]`?
return tokens;
}
// following use cases as working proofs and potential workarounds
// fails typechecking correctly
const token: Token = {
value: 'one',
somethingElse: true // `somethingElse` does not exist on type `Token`
};
// fails typechecking correctly — potential workaround
function getTokens3() {
// note the `Token` inline return type
return things.map((thing): Token => ({
value: thing,
somethingElse: true // `somethingElse` does not exist on type `Token`
}));
}Expected behavior:
I would expect in getTokens1 using the .map<U> generic form that it would typecheck what .map returns. Similarly, I would expect in getTokens2 that it would complain as I am precising the return type of the function and the inference itself seems to show an incompatible type.
Actual behavior:
It doesn't seem to typecheck properly when using Array.map.
Playground Link: See above example in playground
Related Issues: