no-async-promise-executor

Disallow using an async function as a Promise executor

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', function(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:

• 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.

Rule Details

This rule aims to disallow async Promise executor functions.

Examples of incorrect code for this rule:

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

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

Examples of correct code for this rule:

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

const result = Promise.resolve(foo);

When Not To Use It

If your codebase doesn’t support async function syntax, there’s no need to enable this rule.

Version

This rule was introduced in ESLint v5.3.0.

Resources

• Rule source
• Tests source