let a = {
  a: 1,
  b: 2,
  c: 3
};
let pr = Promise.resolve();
for (let key in a) {
  pr.then(() => {
    return new Promise(resolve => {
      setTimeout(() => {
        console.log(key, a[key]);
        resolve();
      }, 500);
    });
  });
}
pr.then(() => {
  console.log('end');
});