Angular 7 unit test with importing JS modules

I am testing a Angular 7 component which has importing JS modules like:

component.ts

import classA from '../../classA'; // Imported JS modules
export class component implements OnInit {
  public a = new classA(10); // Instantiate
  ...
}

classA.js

class classA {
  constructor (a) {
    this.max = a;
    ...
}
module.exports = classA;

component.spec.ts

import classA from '../../classA';

I am importing classA as what I did in component.ts.

This gives an error: TypeError: UndoRedo is not a constructor

I tried to include it in karma.conf.js like:

module.exports = function (config) {
  config.set({
    ...
    files: [
      "../app/classA.js"
    ]
  });
};

But still get same error. Anyone has any idea how to import JS modules in unit testing?