What is the correct way to include one js file in another?

Question

The following JavaScript code works:

let Color = require('color');

class Example {
    its_not_easy_being_green() {
        return Color('green');
    }
}

test('It is not easy being green', () => {
    expect(new Example().its_not_easy_being_green()).toBeDefined();
})

and npx jest gets me a greenbar.

$ npx jest
 PASS  test/example.spec.js
  ✓ It is not easy being green (2 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.351 s, estimated 1 s
Ran all test suites.

However, any time I try to separate the class from the test I get various errors:

The most common is one about not being permitted to use import outside a module.

Or that Example is not a constructor.

What I am trying to do is move everything that isn't the three line test out of the JavaScript file that I've posted above.

If this were Java, I'd move that stuff into a separate .java file and then import that file in my test.

If this were C there would be a linker and #include involved.

What I've tried is to create an Example.js file:

let Color = require('color');

class Example {
    its_not_easy_being_green() {
        return Color('green');
    }
}

and then require it in the test:

let Example = require('Example.js')

test('It is not easy being green', () => {
    expect(new Example().its_not_easy_being_green()).toBeDefined();
})

But that gets me a complaint about Example not being a constructor, which, I suppose is true. It is a class, which presumably has a pretty inert default constructor of some kind.

I've also tried replacing the require with an import:

import('Example.js')

test('It is not easy being green', () => {
    expect(new Example().its_not_easy_being_green()).toBeDefined();
})

But that gets complaints about not being permitted to use import outside a module.

Is it possible to separate the jest test from the code being tested in JavaScript?