await.js

let sessionId;
async function waitOnSign() {
  return new Promise(resolve => {
    let chk = setInterval(() => {
      if (sessionId) {
        clearInterval(chk)
        resolve(sessionId)
      }
    }, 100);
  });
}

async function asyncCall() {
  console.log('calling');
  const result = await waitOnSign();
  console.log(result);
}

asyncCall();

setTimeout(() => {
  sessionId = "xxxxx"
}, 2000);