<pre class="shiki github-dark shaku"><code>class MapPolyFill { constructor(initialValues) { // initial values as per specification should be an array of [key,value] this.entries = []; if(Array.isArray(initialValues) &amp;&amp; initialValues.length &gt; 0) { this.entries = [...initialValues]; } } has(key) { const idx = this.entries.findIndex((entry) =&gt; entry[0] === key); return idx &gt; -1; } get(key) { const entry = this.entries.find((entry) =&gt; entry[0] === key); return entry ? entry[1] : undefined; } set(key,value) { const idx = this.entries.findIndex((entry) =&gt; entry[0] === key); if(idx === -1) { this.entries.push([key,value]) } else { this.entries[idx][1] = value; } return this; }}class NodeStore { /** * @param {Node} node * @param {any} value */ constructor() { this.map = new MapPolyFill(); } set(node, value) { return this.map.set(node,value); } /** * @param {Node} node * @return {any} */ get(node) { return this.map.get(node); } /** * @param {Node} node * @return {Boolean} */ has(node) { return this.map.has(node); }}</code></pre>

Node store - with a Map polyfill class (P.S. classes are not ES5 feature either :) --- better to code such polyfills with transpilers