Defining a TypeScript conditional return type that may include undefined

Here is the function I currently have, but unfortunately I encountered the following error:

Type '(T & undefined) | { obj: T & ({} | null); }' is not assignable to type 'T extends undefined ? undefined : { obj: T; }'. Type 'T & undefined' is not assignable to type 'T extends undefined ? undefined : { obj: T; }'.(2322)

function undefinedOrObject<T>( param: T ): T extends undefined ? undefined: {obj: T} {
    return typeof param === 'undefined' ? param : { obj: param } 
}

param can be of type number, boolean or undefined. If it's undefined, I simply want to return undefined. For any other primitive type, I would like to return an object like {obj: param}. My goal is to achieve this without using any casting with as .... Do you have any suggestions on how to approach this?

typescripttypescript-conditional-types

Answer №1

Shoutout to @jcalz for the insight!

Exploring the idea of overloading as a potential solution:

function undefinedOrObject<T extends (number| boolean | undefined)> (obj: T ): T extends undefined ? undefined : { obj: T }
function undefinedOrObject<T>( param: T ) {
    return typeof param === 'undefined' ? undefined : {obj: param}
}


const u = undefinedOrObject(undefined)
const n = undefinedOrObject(3)
const o = undefinedOrObject({t:7}) // An intentional error occurs here, which is expected

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