Map like structure that can has key is an object or DOM Node.

DOMStore.js

/* Using array to store and search for key. Time complexity O(n) */
class DOMStoreArray {
  constructor () {
    this.data = [];
  }
  
  find (node) {
    for (let item of this.data) {
      if (item[0] === node) {
        return item;
      }
    }
    return null;
  }
  
  has (node) {
    return this.find(node) !== null;
  }
  
  set (node, value) {
    const item = this.find(node);
    if (item) {
      item[1] = value;
      return;
    }
    this.data.push([node, value]);
  }
  
  get (node, defaultValue) {
    const item = this.find(node);
    if (item) {
      return item[1];
    }
    return defaultValue;
  }
}

/* Cheating, store data directly on object O(1)*/
class DOMStore {
  constructor () {
    this.DOMStoreSymbol = Symbol('DOMStore');
  } 
  has (node) {
    return node[this.DOMStoreSymbol] !== undefined;
  }

  set (node, value) {
    node[this.DOMStoreSymbol] = value;
  }

  get (node, defaultValue) {
    return node[this.DOMStoreSymbol] || defaultValue;
  }
}

/* Test */
const e1 = document.createElement('A');
const e2 = document.createElement('P');
const e3 = document.createElement('DIV');
const e4 = document.createElement('TABLE');
const e5 = document.createElement('A');
const map1 = new DOMStore();
const map2 = new DOMStore();

console.assert(map1.has(e1) === false, 'map1 should be empty');
map1.set(e1, 'E1 in map1');
console.assert(map1.has(e1) === true, 'map1 must have e1');
map2.has(e1);
console.assert(map2.has(e1) === false, 'map2 should be empty');
map1.set(e2, 1234);
map1.set(e3, "String");
map1.set(e4, [1,2,3,4]);
console.assert(map1.get(e5, null) === null);
console.assert(map1.get(e5) === undefined);
console.assert(map1.get(e5, 5) === 5);
map1.set(e5, {1: 2, 3: 4});
console.assert(map1.get(e1) === 'E1 in map1');
console.assert(typeof(map1.get(e5)) === 'object');