How to determine the return type based on the quantity of arguments passed to a rest parameter function

Is there a way to create an arrow function using rest parameters that can return different types based on the number of arguments passed?

For example, I am looking to implement a safeId() function with the following return type variations:

safeId() // () => string
safeId('foo') // (obj: any) => string
safeId('foo', 4, {}, ['hi']) // (...obj: any[]) => string[]

// Currently, the return type is not specific:
const safeId = (...objects: any[]) => string | string[] => {
  // Simplified functionality
  if (objects.length > 1) return sanitizedObjectIds(objects) // string[]
  return 'id'
}
typescripttypescript-generics

Answer №1

To ensure smooth functionality, make sure to define all variations of your function and handle them effectively:

function ensureId(id?: string): string
function ensureId(id: string, num: number, ...rest: any[]): string[]
function ensureId(...objects: any[]): string | string[] {
  // Simplified
  if (objects.length > 1) return sanitizedObjectIds(objects) // string[]
  return 'id'
}

It is important to note that adding a second required parameter in the second version of the function eliminates ambiguity. Without it, the single-parameter form could also match (with an empty array for rest).

UPDATE: If you prefer using arrow functions, the compiler may flag the return type issue. In my opinion, this is a flaw in TypeScript. Therefore, the workaround is to bypass type checking by returning any:

type EnsuredId = {
  (id?: string): string
  (id: string, num: number, ...rest: any[]): string[]
}

const ensureId: EnsuredId = (...objects: any[]): any => {
  // Simplified
  if (objects.length > 1) return sanitizedObjectIds(objects) // string[]
  return 'id'
}

Please be aware there is an ongoing issue related to this in the TypeScript repository.

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