In this post, App Dev Manager Ashley Grant shows how to cache data in JavaScript Promises.

JavaScript Promises are powerful, and they become more powerful once we understand that a “resolved” Promise can be used more than once! How can we use this in a “real” application? Maybe you have a button in your application that will display user information when clicked. You could wait to make the fetch call until the button is clicked, and then save this Promise in a variable. In future calls, the code will see that this Promise already exists, and thus skip calling fetch again.

Something like this:

let fetchUsers;   

document.getElementById('displayUser').onclick = () => {
  if(!fetchUsers) {
    fetchUsers = fetch('https://api.github.com/users')
      .then( response => response.json());
  } 
  
  const index = document.getElementById('index').value;
  
  fetchUsers.then(users => {
    document.getElementById('userName').innerHTML = users[index].login;
  });
};

Read the full article here.