no-callback-in-promise.md

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

⚠️ This rule warns in the following configs: ✅ flat/recommended, ✅ recommended.

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

Options

timeoutsErr

Boolean as to whether callbacks in timeout functions like setTimeout will err. Defaults to false.

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.