When the constraint
T extends { [key in keyof Partial<GM>]: 1 }
is applied, it means that T must be able to be assigned to {a?:1, b?:1, c?:1}. This not only includes types you intend to support, like {a: 1}, but also types you may not have intended to support. In TypeScript, object types are considered "extendable" or "open", meaning properties can be added to them. Therefore, a type like
{a?: 1, b?:1, c?:1, oops: string}
is also supported:
const oopsie = getVF({ a: 1, oops: "oops" }) // no error!
// const oopsie: VMeth<GM, Pick<GM, "a" | "oops">
This leads to the compiler rightly complaining about the following:
// Type 'keyof T' does not satisfy the constraint 'keyof GM'.
If you want to specifically limit the keys of part to those of GM, you can make your function generic in those keys using K:
const getVF = <K extends keyof GM>(part: Record<K, 1>):
VMeth<GM, Pick<GM, K>> => {
return function () { } as any
}
Now, K must be a subset of
"a" | "b" | "c"
and cannot include
"oops" or any other unauthorized key. The functionality remains the same for your desired use case:
fd({ a: 1, b: "", c: "s" }, { a: 1 });
// const fd: (old: GM, n: Pick<GM, "a">) => any
You will now receive compiler warnings if an unexpected property is added:
getVF({ a: 1, oops: "oops" }); // error, excess property!
Even though it's still possible to sneak in an excess property by being tricky:
const existingObj = {a: 1, oops: "oops"} as const;
const aliased: {a: 1} = existingObj;
const okay = getVF(aliased); // no error
// const okay: VMeth<GM, Pick<GM, "a">>
At least the resulting value is always Pick<GM, "a"> and not an invalid selection like
Pick<GM, "a" | "oops">
.
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