Get All Properties of a Type with this Fancy Utility Type

Introducing... PropertiesOfType!

Posted on March 15, 2022

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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

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