insert object keys in order, enumerate them in order

1.js

var obj = { a: 1, b: 2, c: 3, d: 4 };

insert(obj,{ property: "g", value: 7 });
insert(obj,{ before: "g", property: "e", value: 5 });
insert(obj,{ property: "h", value: 8 });
insert(obj,{ before: "g", property: "f", value: 6 });
obj.i = 9;

console.log(keys(obj));
// [ "a", "b", "c", "d", "e", "f", "g", "h", "i" ]

delete obj.g;
console.log(keys(obj));
// [ "a", "b", "c", "d", "e", "f", "h", "i" ]

2.js

var { insert, keys } = (function define(){
  var keyStore = new WeakMap();
  return { insert, keys };
  
  function insert(obj,{ before = "", property, value }){
    registerKeys(obj);  // this is optional for safety/flexibility...
    obj[property] = value;
  
    if (!keyStore.has(obj)) {
      keyStore.set(obj,[]);
    }
    var keyOrder = keyStore.get(obj);
    var beforeIdx = keyOrder.findIndex(k => k === before);
    if (beforeIdx >= 0) {
      keyOrder.splice(beforeIdx,0,property);
    }
    else {
      keyOrder.push(property);
    }
  }
  
  function keys(obj) {
    registerKeys(obj);  // this is optional for safety/flexibility...
    return [ ...(keyStore.get(obj) || []) ];
  }
  
  // this is optional for safety/flexibility...
  function registerKeys(obj) {
    var objKeys = Object.keys(obj);
    var knownKeys = keyStore.get(obj) || [];
    keyStore.set(obj,knownKeys);
    
    if (objKeys.length > 0) {
      if (knownKeys.length > 0) {
        // have any keys been removed from the object?
        for (let [idx,key] of knownKeys.entries()) {
          if (!objKeys.includes(key)) {
            knownKeys.splice(idx,1);
          }
        }
      
        // are there any newly-discovered keys in the object?
        for (let [idx,key] of objKeys.entries()) {
          if (!knownKeys.includes(key)) {
            knownKeys.push(key);
          }
        }
      }
      else {
        knownKeys.push(...objKeys);
      }
    }
  }
})();