Getting type of a property of a typescript class using keyof operator

Question

As stated in the documentation of Typescript about the keyof operator, one can get a property of an object instance using the function below.

function getProperty<T, K extends keyof T>(o: T, name: K) {     return o[name]; } 

Of course, one can get the type of the property by replacing return o[name] into return typeof o[name]. Is there a way to retrieve the type of the property without passing any object instance?

function getPropertyType<T>(name: keyof T) {     // something like T[name]? } 

Answer 1

Is this what you're looking for?

type PropType<TObj, TProp extends keyof TObj> = TObj[TProp]; 

and get type of an object property by doing:

type MyPropType = PropType<ObjType, '<key>'>; 

which is the same as the way of using Pick in typescript, and it can report compile error if there's any invalid key passed in.

Updates

As @astoilkov suggested, a simpler alternative is PropType['key'].

Answer 2

Yes, lookup types work just fine:

type BarType = FooType['bar']; 

It expects in this case that FooType is an object like:

type FooType = {     bar: string; } 

It sets BarType to the same type as FooType['bar'], so to a string.

PS: FooType can also be an interface or class.