Suppose I have this interface
interface MyInterface {
val1: string
val2: string
val3: number
val4: string
val5: Date
}
I want to make a type that is the keys of the MyInterface but only the keys that are strings:
type myType = keyof MyInterface //where key === typeof 'string'
// myType: 'val1' | 'val2' | 'val4'
How could I do something like this?
You can use a mapped type to preserve only specif keys and then do a keyof on the resulting type
type myType = keyof {
[P in keyof MyInterface as MyInterface[P] extends string ? P: never]: any
}
You could also make it generic
type KeyOfType<T, V> = keyof {
[P in keyof T as T[P] extends V? P: never]: any
}
type myType = KeyOfType<MyInterface, string>
type myType2 = KeyOfType<MyInterface, number>
The as clause of a mapped type allows us to manipulate the name of the key. If the key is mapped to never the key gets removed from the resulting type. You can find more info in this PR
type KeyOfType<T, V> = { [P in keyof T]: T[P] extends V ? P: never }[keyof T]