Using TypeScript's generic rest parameters to form a union return type

Right now, in TypeScript you can define dynamic generic parameters.

function bind<U extends any[]>(...args: U);

However, is it possible for a function to return a union of argument types? For example:

function bind<U extends any[]>(...args: U): U1 | U2 | U3...;

Is there a method to achieve this?

Answer №1

If you want to combine all arguments together, you can utilize the U[number] method:

function merge<U extends any[]>(...args: U): U[number] {
    return args[Math.round(Math.random()*(args.length - 1))]; // example implementation
}
let result = merge(1,"2", true) // number | string | boolean
console.log(result)

You also have the option to retrieve the type at a specific position, but because the position may not always be present, a conditional type is necessary;

type GetAt<T extends any[], I extends number> = T extends Record<I, infer U> ? U : never;
function merge<U extends any[]>(...args: U): GetAt<U, 0> {
    return args[Math.round(Math.random()*(args.length - 1))]; // example implementation
}
let output = merge(1,"2", true) // number

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