Unexpected identifier when exporting typescript class

Question

I have a typescript class called Polynomial, and I want to create a npm module with it. For that, I'm trying to test it with mocha, with this script:

//Polynomial.test.js
const Polynomial = require('./Polynomial.ts'); 
const assert = require('assert').strict;

describe("First test", function() {
    it("test", function() {
        let p = new Polynomial();
        assert(p.equals(new Polynomial("0")));
    });
});

When I do npm test I get the error Unexpected identifier at the very first line of the class, which look like:

// Polynomial.ts
class Polynomial {
  private coefMap: Map<string, string> = new Map();  // <--- Error
  // ...
}
module.exports = Polynomial;

This is the package.json:

{
  "name": "multivariate-polynomial",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
    "test": "mocha Polynomial.test.js"
  }
}

And the tsconfig.json:

{
    "compilerOptions": {
      "target": "es5",
      "lib": [
        "dom",
        "dom.iterable",
        "esnext"
      ],
      "module": "commonjs",
      "declaration": true,
      "outDir": "./lib",
      "strict": true
    },
  }

Any hint in what the problem is?