undefined is not supported in model properties

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Expected Behavior

I would like to use models that can have the undefined type:

import {Controller, Get, Route} from 'tsoa';

interface User {
  name: string | undefined;
}

@Route('user')
export class UserController extends Controller {
  @Get()
  public async getUser(): Promise<User> {
    return {name: undefined};
  }
}

Current Behavior

I get an error when trying to generate the spec:

yarn tsoa spec
yarn run v1.22.4
$ /Users/cedric/dev/kraaft-firebase/app-engine/node_modules/.bin/tsoa spec
There was a problem resolving type of 'User'.
Generate swagger error.
 Error: Unknown type: UndefinedKeyword
At: src/api/userController.ts:10:10.
This was caused by 'string | undefined'
    at new GenerateMetadataError (/Users/cedric/dev/kraaft-firebase/app-engine/node_modules/tsoa/node_modules/@tsoa/cli/dist/metadataGeneration/exceptions.js:22:28)
    at TypeResolver.resolve (/Users/cedric/dev/kraaft-firebase/app-engine/node_modules/tsoa/node_modules/@tsoa/cli/dist/metadataGeneration/typeResolver.js:308:19)
    at /Users/cedric/dev/kraaft-firebase/app-engine/node_modules/tsoa/node_modules/@tsoa/cli/dist/metadataGeneration/typeResolver.js:113:95
    at Array.map (<anonymous>)
    at TypeResolver.resolve (/Users/cedric/dev/kraaft-firebase/app-engine/node_modules/tsoa/node_modules/@tsoa/cli/dist/metadataGeneration/typeResolver.js:112:45)
    at TypeResolver.propertyFromSignature (/Users/cedric/dev/kraaft-firebase/app-engine/node_modules/tsoa/node_modules/@tsoa/cli/dist/metadataGeneration/typeResolver.js:761:143)
    at /Users/cedric/dev/kraaft-firebase/app-engine/node_modules/tsoa/node_modules/@tsoa/cli/dist/metadataGeneration/typeResolver.js:727:161
    at Array.map (<anonymous>)
    at TypeResolver.getModelProperties (/Users/cedric/dev/kraaft-firebase/app-engine/node_modules/tsoa/node_modules/@tsoa/cli/dist/metadataGeneration/typeResolver.js:727:124)
    at TypeResolver.getModelReference (/Users/cedric/dev/kraaft-firebase/app-engine/node_modules/tsoa/node_modules/@tsoa/cli/dist/metadataGeneration/typeResolver.js:552:31)

Possible Solution

The attribute could be treated as if it was optional.

Steps to Reproduce

1. create a controller like the one above
2. generate the spec: yarn tsoa spec

Context (Environment)

Version of the library: 3.4.0 Version of NodeJS: v10.21.0

• Confirm you were using yarn not npm: [x]

Detailed Description

Breaking change?

No

[WoH]

OpenAPI 2/3 does not support an undefined type, only optional properties. We still want to distinguish between null and undefined, so modeling this in TS using

interface User {
  name?: string;
}

is the best way to represent this currently. The proper way to solve this would be to add a Tsoa.UndefinedType, which marks the property as optional if it is a property type and handles the case where it is not, for example a route handler: public getStuff(): undefined.

[haedaal]

Ran into same issue. Anyone holding this issue?