singleton.js

const singleton = (function() {
  let instance;

  function init() {
    let count = 0;

    return {
      getValue,
      increase
    };

    function getValue() {
      return count;
    }

    function increase() {
      return count += 1;
    }
  }

  return {
    getInstance() {
      !instance && (instance = init());
      return instance;
    },
  };
})();

var s = singleton.getInstance();
console.log(s.getValue()); // 0
console.log(s.increase()); // 1

var s2 = singleton.getInstance();
console.log(s2.getValue()); // 1
console.log(s2 === s); // true