https://github.com/istanbuljs/nyc/issues/1209

.gitignore

.nyc_output
coverage
node_modules
package-lock.json

main.ts

export class Main {
  public x: number;

  constructor() {
    this.x = 1;
  }
}

export namespace Main {
  export function f() {
    return 2;
  }
}

package.json

{
  "private": true,
  "scripts": {
    "test": "nyc mocha -r ts-node/register test.ts"
  },
  "nyc": {
    "extension": [
      ".ts"
    ],
    "exclude": [
      "test.ts"
    ]
  },
  "devDependencies": {
    "@types/chai": "^4.2.3",
    "@types/mocha": "^5.2.7",
    "chai": "^4.2.0",
    "mocha": "^6.2.2",
    "nyc": "^14.1.1",
    "source-map-support": "^0.5.13",
    "ts-node": "^8.4.1",
    "typescript": "^3.6.4"
  }
}

test.ts

import { expect } from "chai";

import { Main } from "./main";

describe("Main type", () => {
  it("is a class", () => {
    const m = new Main();
    expect(m.x).eq(1);
  });

  it("has a function", () => {
    const v = Main.f();
    expect(v).eq(2);
  });
});

tsconfig.json

{
  "compilerOptions": {
    "emitDecoratorMetadata": true,
    "esModuleInterop": true,
    "experimentalDecorators": true,
    "forceConsistentCasingInFileNames": true,
    "module": "commonjs",
    "noImplicitAny": false,
    "noImplicitReturns": true,
    "noUnusedLocals": true,
    "sourceMap": true,
    "strict": true,
    "target": "es2019",
    "types": [
      "mocha"
    ]
  },
  "include": [
    "*.ts"
  ],
  "exclude": [
    "**/node_modules"
  ]
}