TypeScript: Declaring union types for objects without specifying the entire type for each one

Currently, I am in the process of converting frontend Java code to TypeScript and one challenge I am facing is dealing with method overloading.

I originally wanted to define the types like this:

const person = (info: {name:string, age:number} | {man:boolean}): void => {
    if (typeof info.man === "boolean") {
        console.log("man is defined", info.man);
    } else {
        console.log(info.name, info.age);
    }
}

person({name:"Joe", age:25});
person({ man: false});

However, to satisfy TypeScript, it ended up looking like this:

const person = (info: {name:string, age:number, man?:undefined} 
  | {name?:string, age?:number, man:boolean}): void => {
    if (typeof info.man === "boolean") {
        console.log("man is defined", info.man);
    } else {
        console.log(info.name, info.age);
    }
}

person({name:"Joe", age:25});
person({ man: false});

I am really hoping that there exists a cleaner and more readable way to define these types.

Answer №1

In TypeScript, method overloading is supported by defining multiple function signatures. The compiler will hide all but the last signature, which represents the actual implementation.

function person(info: { name: string, age: number }): void;
function person(info: { man: boolean }): void;
function person(info: any) {
    if (typeof info.man === "boolean") {
        console.log("man is defined", info.man);
    } else {
        console.log(info.name, info.age);
    }
}

If using an arrow function is preferred, the overloads can be defined in a separate type.

type Person = {
    (info: { man: boolean }): void,
    (info: { name: string, age: number}): void,
}
const person: Person = (info: any) => {
    if (typeof info.man === "boolean") {
        console.log("man is defined", info.man);
    } else {
        console.log(info.name, info.age);
    }
}

Answer №2

Consider utilizing default function overloading in this scenario. While it may not match your exact example, it offers enhanced readability:

function individual(info: { male: boolean }): void;
function individual(info: { name: string, years: number }): void;
function individual(info: any): void {
    if (typeof info.male === "boolean") {
        console.log("The individual is male:", info.male);
    } else {
        console.log(info.name, info.years);
    }
}

This approach allows the compiler to use overloaded signatures for type checking while concealing the implementation's signature.

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