instanceof not work correctly in typescript library project

Question

I build a typescript library project. and use some classes from a public package.
let's say a simple function to test it is a BadRequestException or not

import { BadRequestException } from "@nestjs/common";
export function test(error) {
  let a = error;
  let b = BadRequestException;
  console.log(a instanceof b);
}

after build and import this shared-lib library from another normal project and i called like this

import { BadRequestException } from "@nestjs/common";
import { test } from 'shared-lib';
test(new BadRequestException('it is a test error'));

in my throught(it should be), a instance b in test function should be equals true,however,i got false

is that BadRequestException used in the library project is different from that in the second project, even both of them are imported from "@nestjs/common"?

uploaded a demo project in Github

Answer 1

instanceof checks if the constructors are the same. The thing is that your code gets its own copy of the constructor from the code in the library (because that's how it is, it's not your fault), so they are not the same instance anymore.

The constructors still share the same name though:

error.constructor.name === BadRequestException.prototype.constructor.name

See related question here

Answer 2

Using the accepted answer, you can create a narrowing function using type predicates.

E.g.:

const isBadRequestException = (error: any): error is BadRequestException => {
  return error.constructor.name === BadRequestException.prototype.constructor.name;
}

Then you can use this for subsequent typed access to the error object:

if (isBadRequestException(error)) {
  error... // Properties now available
}