counter.js

let count = undefined;
let startTime = undefined;

export class ErrorNotStarted extends Error {
  constructor() {
    super("The counter has not been started.");
  }
}

export class ErrorAlreadyStarted extends Error {
  constructor() {
    super("The counter has already been started.");
  }
}

export function next() {
  ensureStarted();
  return count++;
}

export function start() {
  ensureStarted(false);
  startTime = new Date();
  return count = 0;
}

export function timeSinceStart() {
  ensureStarted();
  return startTime;
}

function ensureStarted(wantStarted = true) {
  if (startTime === undefined && wantStarted) {
    throw new ErrorNotStarted();
  }
  if (startTime !== undefined && !wantStarted) {
    throw new ErrorAlreadyStarted();
  }
}

uses-counter.js

import { start, next, timeSinceStart } from 'counter.js';
// import { count, startTime } from 'counter.js'; // This is a language-level error.

function handleStart() {
  start();
}

function handleClick() {
  next();
}