title: React/Typescript multimap
body: <p>I'm new to typescript and not able to figure out how to implement a multimap. I have the code as below. I need to iterate through the itemArray and store the Item according to date. I need to use the date as the key to the multimap. How to go about implementing this without using any external libraries?</p>
  <pre><code>interface Item {
      id: number;
      date: string;
  }
  
  interface Details{
      itemArray: Item[]
  }
  </code></pre>