Get All Properties of a Type with this Fancy Utility Type
Introducing... PropertiesOfType!
Posted on March 15, 2022
Let's say we've got a function for which only comparison operators make sense. In this case, a function that compares a given property of two objects with the same type and tells you if the first is greater than the second:
function greaterThan<T>(a: T, b: T, property: any): boolean {
return a[property] > b[property];
}
Instead of that ugly looking (and incorrect) any type for property, we should instead allow only the types that make sense for the greater than (>) operator: string, Date, and number.
Introducing PropertiesOfType
I introduce to you the type util, PropertiesOfType:
type PropertiesOfType<Type, ValueTypes> = { [K in keyof Type]: Type[K] extends ValueTypes ? K : never }[keyof Type];
This type can select the keys of any type according to the value of ValueTypes. Because TypeScript is amazing, Type can be generic! So for our example, we could use:
property: PropertiesOfType<T, string | Date | number>
Or incorporated into greaterThan:
function greaterThan<T>(a: T, b: T, property: PropertiesOfType<T, >): boolean {
return a[property] > b[property];
}
and we're all set!
Many thanks to carlvincentld on the TypeScript GitHub for clarifying how Extract works and providing me with this solution!
Thanks!
Chris
