I want to be able to assign an object property to a value given a key and value as inputs yet still be able to determine the type of the value. It's a bit hard to explain so this code should reveal the problem:
type JWT = { id: string, token: string, expire: Date };
const obj: JWT = { id: 'abc123', token: 'tk01', expire: new Date(2018, 2, 14) };
function print(key: keyof JWT) {
switch (key) {
case 'id':
case 'token':
console.log(obj[key].toUpperCase());
break;
case 'expire':
console.log(obj[key].toISOString());
break;
}
}
function onChange(key: keyof JWT, value: any) {
switch (key) {
case 'id':
case 'token':
obj[key] = value + ' (assigned)';
break;
case 'expire':
obj[key] = value;
break;
}
}
print('id');
print('expire');
onChange('id', 'def456');
onChange('expire', new Date(2018, 3, 14));
print('id');
print('expire');
onChange('expire', 1337); // should fail here at compile time
print('expire'); // actually fails here at run time
I tried changing value: any to value: valueof JWT but that didn't work.
Ideally, onChange('expire', 1337) would fail because 1337 is not a Date type.
How can I change value: any to be the value of the given key?
UPDATE: Looks like the question title attracts people looking for a union of all possible property value types, analogous to the way keyof gives you the union of all possible property key types. Let's help those people first. You can make a ValueOf analogous to keyof, by using indexed access types with keyof T as the key, like so:
type ValueOf<T> = T[keyof T];
which gives you
type Foo = { a: string, b: number };
type ValueOfFoo = ValueOf<Foo>; // string | number
ALSO: If you're creating an object literal and need the union of those property values as literal types like "0" instead of string, then you might want to use a const assertion; see @Dima's answer.
For the question as stated, you can use individual keys, narrower than keyof T, to extract just the value type you care about:
type sameAsString = Foo['a']; // look up a in Foo
type sameAsNumber = Foo['b']; // look up b in Foo
In order to make sure that the key/value pair "match up" properly in a function, you should use generics as well as indexed access types, like this:
declare function onChange<K extends keyof JWT>(key: K, value: JWT[K]): void;
onChange('id', 'def456'); // okay
onChange('expire', new Date(2018, 3, 14)); // okay
onChange('expire', 1337); // error. 1337 not assignable to Date
The idea is that the key parameter allows the compiler to infer the generic K parameter. Then it requires that value matches JWT[K], the indexed access type you need.