Use `localStorage` to Save Username and Pasword

cb.js

// this assumes that you have jquery, or any ajax utils like `isomorphic-fetch`

function storeDataCb(username, password) {
  if (!window.localStorage) return
  
  localStorage.username = username
  localStorage.password = password
}

function logIn() {
  // check if the info has already been cached
  if (localStorage.username && localStorage.password) {
    // fill up the data using localStorage
  }
}

$.ajax({
  // url, body, etc
  success: (data) => {
    storeDataCb(data.username, data.password)
  }
})