How to check if string in type [duplicate]

Question

I have the type:

export type PermissionType = 'creator' | 'editor' | 'viewer' 

At run time, how to check if a variable userInput is actually one the type above

let userInput = 'foo' //
isOfTypePermission(userInput)  // should return false or throw an error

let userInput2 = 'creator'
isOfTypePermission(userInput2) // should return true

In other word, how to get the type PermissionType compiled into a javascript array so that I can just do indexOf(userInput) > -1

Answer 1

Don't over-complicate it.

function isOfTypePermission (userInput: string): userInput is PermissionType {
  return ['creator', 'editor', 'viewer'].includes(userInput);
}

See What does the `is` keyword do in typescript? for more information on why we don't just use a boolean return type.

If your PermissionType is really long, then it might be worth it to infer the type from a const value instead.

const permissions = ['creator', 'editor', 'viewer'] as const;
type PermissionType = (typeof permissions)[number];

function isOfTypePermission (userInput: string): userInput is PermissionType {
  return (permissions as readonly string[]).includes(userInput);
}

Or possibly even a Set

const permissions = new Set(['creator', 'editor', 'viewer'] as const);
type PermissionType = typeof permissions extends Set<infer T> ? T : never;

function isOfTypePermission (userInput: string): userInput is PermissionType {
  return (permissions as Set<string>).has(userInput);
}