javascript async and setInterval for polling

I want most understandable syntax for polling a flag and return when it is true, my code snippet below doesn't work I know, what's the syntax that would make it work if you get my idea ?

async function watch(flag) {
    let id = setInterval(function(){
        if (flag === true) {
            clearInterval(id);
        }
    }, 1000);
    return flag;
}

• If you want to poll a variable where the value is a primative, then you need to define it outside the function, otherwise it can't change.
• If you want to have a promise resolve when that condition is done, then you have to create it explicitly. async and await are tools for managing existing promises.

let flag = false;

function watchFlag() {
  return new Promise(resolve => {
    let i = setInterval(() => {
      console.log("Polling…");
      if (flag) {
        resolve();
        clearInterval(i);
      }
    }, 500);
  });
}

setTimeout(() => {
  flag = true;
}, 1500);

console.log("Watching the flag");

watchFlag().then(() => {
  console.log("The flag has changed");
});