如何检查类定义中方法返回值的类型?

举下面的例子 按钮.js ,其中 javascript class

import htmlToElement from './htmlToElement';

class Button {
  constructor() {
    this._html = htmlToElement('<button>simple html</button>');
  }

  render() {
   return this._html;
  }
}

在 ./按钮.spec.js

describe('Rectangle...', function() {
  it(`should be a class constructor..`, function() {
    // how to test against a specific type?
    expect(new Button().render()).to.be('HTMLButtonElement');
  });
});
// note: mocha and chai are included, jsdom is used also. So the button is rendering.