Let's assume I have an input which adheres to a defined type:
interface Person {
name: string;
age: number;
}
Now, I want a function to accept an array of key-value pairs, eg:
function acceptsPersonProperties(tuple: Array<PersonTuple>) { ... }
// this should be fine:
acceptsPersonProperties([['name', 'Bob'], ['age', 42]]);
// this should give a compile-time error:
acceptsPersonProperties([['name', 2], ['age', 'Bob']]);
Of course, I can type this manually, eg:
type PersonTuple = ['name', string] | ['age', number];
But if type (eg Person
) is a template variable, how can the tuple be expressed as a mapped type ?
function acceptsPropertiesOfT<T>(tuple: Array<MappedTypeHere<T>>) { ... }
To avoid an XY-Problem, the real use case is this:
let request = api.get({
url: 'folder/1/files',
query: [
['fileid', 23],
['fileid', 47],
['fileid', 69]
]
});
which resolves to "/api/folder/1/files?fileid=23&fileid=47&fileid=69"
, but which I want to type, so it does not allow extra properties ( file_id
) and checks types (no string as fileid
).
You can't do it with tuple types. The right side of the tuple will always get generalized to a union of all possible values in Person
.
You can, however, make it work if you change your API slightly:
interface Person {
name: string;
age: number;
}
function acceptsPersonProperties(tuple: Partial<Person>[]) { }
// this should be fine:
acceptsPersonProperties([{ name: 'Bob' }, { age: 42 }]);
// this should give a compile-time error:
acceptsPersonProperties([{ name: 2 }, { age: 'Bob' }]);
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