detect-metamask-locked.js

let account = getAccount();
let accountInterval = setInterval(() => {
  if (getAccount() !== account) {
    account = getAccount();
    if (account === undefined) {
      // if account changed to undefined, that means metamask is now locked
      updateEverything();
    } else {
      // if account is not undefined, that means metamask is now unlocked
      updateEverything();
    }
  }
}, 1000);