Error "Argument of type is not Assignable to" in function with Rest parameters in Typescript

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I got a guy Role which must be a roles object key and a function that returns all keys:

export type Role = keyof typeof roles;

export function allRoles(): Role[] {
  return Object.keys(roles) as Role[];
}

And the next decorator:

export const Roles = (...roles: Role[]) => setMetadata('roles', roles);

However, when trying to use the decorator as @Roles(allRoles()), Typescript informs that there is error in the types:

Argument of type '('Role1', 'Role2')' is not assignable to the parameter of type 'Role1 | Role2'.

typescript typing

  • What is this function Roles? Could [Edit] the question to include your signature?

  • I added, the Roles function simply adds roles in the Multimedia

  • What do you want Roles receive, in fact? An array whose elements are the keys of roles?

  • Exactly, Roles must receive an array of Roles, the items in this array are keys of an object, which in this case is the keyof typeof roles

1 answer

4


The problem is that the guy Role[] is different from the type expected by the construction (...roles: Role[]).

The guy Role[] means any array whose elements are attributable to the type Role.

Already (...roles: Role[]) is a syntactic construct that is used to define the type of a function that accepts parameters Rest, that is, a function with aridity varied.

This means that the function does not accept an array in the first argument (as you are passing), but rather a varied number of arguments of the type Role which together form an array of the type Role[].

Then you need to change the type of the function so that it is no longer a variadic function:

function Roles(...roles: Role[]) {
  // ...
}

Making it a function that accepts only one argument - an array of Roles:

function Roles(roles: Role[]) {
  // ...
}

See working on Typescript playground.

Another option would be to use Function.prototype.apply or the spreading operator at the return of getRoles(), but semantically speaking, I do not believe they are correct. Changing the expected type of function, in this case, seems to me the most appropriate.

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