The mapsy
utility function is used to map a value to a corresponding function based on the type of the value.
To illustrate better how it works imagine we had the following types:
interface Person<T extends Vehicle> {
name: string;
age: number;
vehicle: T;
}
type Vehicle = Car | Bike;
interface Car {
details: {
type: "car";
fuel: "electric" | "gas";
model: string;
year: number;
};
performance: {
horsepower: number;
topSpeed: number; // in km/h
};
}
interface Bike {
details: {
type: "bike";
gear: "fixed" | "multi";
brand: string;
};
specifications: {
weight: number; // in kg
frameMaterial: string;
};
}
If we wanted to create a function called describePerson
that receives a person as
argument and returns different data depending on the vehicle that the person has we
could use the mapsy
function the following way:
import { mapsy } from "mapsy";
// ...
const describePerson = mapsy<{
object: Person<Vehicle>;
nested: ["vehicle"];
subset: ["details", "type"];
params: [];
return: string;
}>(["vehicle"], ["details", "type"], {
car: (person: Person<Car>) => {
return `This person drives a ${person.vehicle.details.fuel} car.`;
},
bike: (person: Person<Bike>) => {
return `This person rides a ${person.vehicle.details.gear} bike.`;
},
});
const personWithCar: Person<Car> = {
name: "John",
age: 25,
vehicle: {
details: {
type: "car",
fuel: "gas",
model: "Toyota Corolla",
year: 2020,
},
performance: {
horsepower: 132,
topSpeed: 180,
},
},
};
console.log(describePerson(personWithCar)); // This person drives a gas car.
The type that the function receives is an object with the following properties:
-
object
: The object type we are dealing with. -
nested
: The path to the key that we want to narrow down. -
subset
: The path to the discriminator key used to extract the subset. -
params
: The extra parameters of the returned function. -
return
: The return type of the returned function.
The function parameters that the functions receives are the following:
- The path to the key that we want to narrow down.
- The path to the discriminator key used to extract the subset.
- An object with the corresponding functions.
If the Person
type was not generic we would need to use the Narrow
utility
type as you can see here:
import { mapsy, Narrow } from "mapsy";
interface Person {
name: string;
age: number;
vehicle: Vehicle;
}
// ...
type PersonWithCar = Narrow<{
object: Person;
nested: ["vehicle"];
subset: ["details", "type"];
reduce: "car";
}>;
type PersonWithBike = Narrow<{
object: Person;
nested: ["vehicle"];
subset: ["details", "type"];
reduce: "bike";
}>;
const describePerson = mapsy<{
object: Person;
nested: ["vehicle"];
subset: ["details", "type"];
params: [];
return: string;
}>(["vehicle"], ["details", "type"], {
car: (person: PersonWithCar) => {
return `This person drives a ${person.vehicle.details.fuel} car.`;
},
bike: (person: PersonWithBike) => {
return `This person rides a ${person.vehicle.details.gear} bike.`;
},
});
const personWithCar: PersonWithCar = {
name: "John",
age: 25,
vehicle: {
details: {
type: "car",
fuel: "gas",
model: "Toyota Corolla",
year: 2020,
},
performance: {
horsepower: 132,
topSpeed: 180,
},
},
};
console.log(describePerson(personWithCar)); // This person drives a gas car.