The Module Pattern from Learning JavaScript Design Patterns by Addy Osmani

index.js

const testModule = (() => {
 
  let counter = 0;
 
  return {
 
    incrementCounter() {
      return counter++;
    },
 
    resetCounter() {
      console.log( `counter value prior to reset: ${counter}` );
      counter = 0;
    }
  };
 
})();
 
// Usage:
 
// Increment our counter
testModule.incrementCounter();
 
// Check the counter value and reset
// Outputs: counter value prior to reset: 1
testModule.resetCounter();