How do you unit test a custom param decorator?

I’m submitting a…

[ ] Regression 
[ ] Bug report
[ ] Feature request
[ X ] Documentation issue or request
[ ] Support request => Please do not submit support request here, instead post your question on Stack Overflow.

Current behavior

How can I test the result of a parameter decorator created using createParamDecorator from @nestjs/common?

Example

Given the following simple decorator how can I test that it correctly returns “user” from the req object?

export const CurrentUser = createParamDecorator((_, req) => req.user);

I know one option would be to separate the factory function that I’m passing in and just test that, but I’d really rather not export it if that’s possible.

[itslenny]

I figured out a solution from digging through the createParamDecorator that source. Seems to work pretty well. Is there a better way of doing this?

function getParamDecoratorFactory(decorator: Function) {
  class Test {
    public test(@decorator() value) { }
  }

  const args = Reflect.getMetadata(ROUTE_ARGS_METADATA, Test, 'test');
  return args[Object.keys(args)[0]].factory;
}

/// Usage

const factory = getParamDecoratorFactory(CurrentUser);
const mockUser = {};
const result = factory(null, {user: mockUser));

expect(result).toBe(mockUser);

[danieldaeschle]

import { ROUTE_ARGS_METADATA } from '@nestjs/common/constants';