Basic implementation of Enum of typescript in vanila javascript.

Enum.js

class Enum {
  constructor(args, startIndex = 0, defaultValue) {
    const em = args.split("|").reduce((m, key, index) => {
      m[(m[key] = index + startIndex)] = key;
      return m;
    }, {});
    em.value = em.key = (k) => em[k];
    return new Proxy(em, {
      get: (obj, prop) => (prop in obj ? obj[prop] : defaultValue),
      set: (obj, prop, value) => {
        if (prop in obj) {
          throw new Error("cant set same value again"); //  // cant set same value again
        }
        em[(em[prop] = value)] = prop;
      },
    });
  }
}
const DOWNLOAD_STATES = new Enum("START|INPROGRESS|FINISHED", 1);
console.log(JSON.stringify(DOWNLOAD_STATES));
// {"1":"START","2":"INPROGRESS","3":"FINISHED","START":1,"INPROGRESS":2,"FINISHED":3}

const { START } = DOWNLOAD_STATES;
console.log(`CURR STATE is ${START}`);
// CURR STATE is 1
console.log(`CURR STATE is ${DOWNLOAD_STATES[2]}`);
// CURR STATE is INPROGRESS

// ADD new State:
DOWNLOAD_STATES.IDLE = 4;
console.log(`CURR STATE is ${DOWNLOAD_STATES[4]}`);

// CURR STATE is IDLE