Illegal return statement when importing class from Node.js

I’m running Node.js from VS Code. I copied the Color class from this link: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Using_classes

Here is the color.mjs:

export class Color {
  #values;
  // …
  toString() {
    return this.#values.join(", ");
  }
}

And then, I import this class from app.js:

import { Color } from "./color.mjs";

Here is the error:

Uncaught SyntaxError SyntaxError: Illegal return statement
    at compileSourceTextModule (<node_internals>/internal/modules/esm/utils:338:16)
    at moduleStrategy (<node_internals>/internal/modules/esm/translators:102:18)
    at #translate (<node_internals>/internal/modules/esm/loader:468:12)
    at loadAndTranslate (<node_internals>/internal/modules/esm/loader:515:27)
    --- await ---
    at runEntryPointWithESMLoader (<node_internals>/internal/modules/run_main:138:19)
    at loadESMFromCJS (<node_internals>/internal/modules/cjs/loader:1329:42)
    at <anonymous> (<node_internals>/internal/modules/cjs/loader:1536:5)
    at <anonymous> (<node_internals>/internal/modules/cjs/loader:1706:10)
    at <anonymous> (<node_internals>/internal/modules/cjs/loader:1289:32)
    at <anonymous> (<node_internals>/internal/modules/cjs/loader:1108:12)
    at traceSync (<node_internals>/diagnostics_channel:322:14)
    at wrapModuleLoad (<node_internals>/internal/modules/cjs/loader:220:24)
    at executeUserEntryPoint (<node_internals>/internal/modules/run_main:170:5)
    at <anonymous> (<node_internals>/internal/main/run_main_module:36:49)
utils:338
No debugger available, can not send 'variables'

Why can’t I import class in app.js? What’s missing in my code? And how to fix?