no-callback-in-promise.md

Disallow calling cb() inside of a then() (use nodeify instead) (promise/no-callback-in-promise)

⚠️ This rule warns in the ✅ recommended config.

As a general rule, callbacks should never be directly invoked inside a Promise.prototype.then() or Promise.prototype.catch() method. That's because your callback may be unintentionally be invoked twice. It also can be confusing to mix paradigms.

Take the following example:

function callback(err, data) {
  console.log('Callback got called with:', err, data)
  throw new Error('My error')
}

// note: passing `err.message` for demo purposes, normally you would pass `err`
Promise.resolve()
  .then(() => callback(null, 'data'))
  .catch((err) => callback(err.message, null))

If you run this example, your output will look like the following:

Callback got called with: null data
Callback got called with: My error null

How to fix it?

Ensure that your callback invocations are wrapped by a deferred execution function such as:

• setImmediate() or process.nextTick(): for Node.js.
• setTimeout(): for Browsers and Node.js.

// node.js
Promise.resolve()
  .then(() => setImmediate(() => callback(null, 'data')))
  .catch((err) => setImmediate(() => callback(err.message, null)))

// node.js and browsers
Promise.resolve()
  .then(() => setTimeout(() => callback(null, 'data'), 0))
  .catch((err) => setTimeout(() => callback(err.message, null), 0))

Your output will now look like the following:

Callback got called with: null data

Finally, if your callbacks have a Node.js signature (i.e. callback(err, data)), consider using util.promisify for promisifying your callback code instead of combining the approaches.

Options

exceptions

String list of callback function names to exempt.