The new Promise constructor accepts an executor function as an argument, which has resolve and reject parameters that can be used to control the state of the created Promise. For example:

const result = new Promise(function executor(resolve, reject) {
  readFile('foo.txt', (err, result) => {
    if (err) {
      reject(err);
    } else {
      resolve(result);
    }
  });
});

The executor function can also be an async function. However, this is usually a mistake, for a few reasons:

• The Promise constructor can only catch synchronous exceptions, and an async function never throws - it always returns a promise (which might get rejected though). And that return value is ignored, as the promise is waiting for resolve to be called.
• Therefore, if an async executor function throws an error, the error will be lost and won't cause the newly-constructed Promise to reject. This could make it difficult to debug and handle some errors.
• If a Promise executor function is using await, this is usually a sign that it is not actually necessary to use the new Promise constructor, or the scope of the new Promise constructor can be reduced.

Bad Practice

const foo = new Promise(async (resolve, reject) => {
  readFile('foo.txt', (err, result) => {
    if (err) {
      reject(err);
    } else {
      resolve(result);
    }
  });
});

const result = new Promise(async (resolve, reject) => {
  resolve(await foo);
});

Recommended

const foo = new Promise((resolve, reject) => {
  readFile('foo.txt', function(err, result) {
    if (err) {
      reject(err);
    } else {
      resolve(result);
    }
  });
});

const result = Promise.resolve(foo);