What could this be about? [TsLint Error: "Promises must be handled appropriately"]

Question

I'm doing some basic asynchronous operations using async/await in TypeScript but TSLint is throwing mysterious error messages for these two functions below. Has anyone encountered these errors before? On the error output the governing rule is not mentioned, so I don't understand what's causing these. Any ideas would be greatly appreciated.

The main request:

import * as rp from 'request-promise'

export function getRequest(address: rp.Options): rp.RequestPromise {
  return rp(address)
}

Exported async function:

export async function getStatus(message: Message) {
  try {
    const res = await getRequest(address)
    if (res.ready) {
      message.reply('...')
    } else {
      message.reply('...')
    }
  } catch (err) {
    message.reply(err)
  }
}

This gets: Promises must be handled appropriatelyand await of non-Promise for line #3.

The simple function that uses this export is:

client.on('message', message => {
  if (message.content === 'green') {
    getStatus(message)
  }
})

This also gets Promises must be handled appropriately.

Additional information:

Even though the error message doesn't mention it, this seems to be the governing rule for Promises must be handled appropriately: https://palantir.github.io/tslint/rules/no-floating-promises/

And this Issue mentions await of non-Promise: https://github.com/palantir/tslint/issues/2661

Answer 1

That's a crappy error message. A better one might be,

every expression of type Promise must end with a call to .catch or a call to .then with a rejection handler (source).

So, for example, if you do

PromiseFunction()
  .catch(err => handle(err))
  .then(() => console.log('this will succeed'))

then you will still have a tslint problem, because the type of .then(...) is a promise, and it has to end with a catch. The fix would be appending a .catch clause, for example,

PromiseFunction()
  .catch(err => handle(err))
  .then(() => console.log('this will succeed'))
  .catch(() => 'obligatory catch')

or just disabling tslint for that line via:

PromiseFunction()
  .catch(err => handle(err))
  // tslint:disable-next-line:no-unsafe-any
  .then(() => console.log('this will succeed'))

Alternatively, you could reverse the order of the .then and .catch statements. However, that stops the .then from executing if an error does occur, which you presumably want if you encountered this problem.

Answer 2

Sometimes you might want to call the promise, but you don't need to do anything with the response. A route change or something else.

so instead of:

promiseFunction().then().catch()
try/catch async/await

you can do:

void promiseFunction();

As per the comments on the correct use of void please read: IgnoreVoid

Use at your own convenience :)

Answer 3

I have got the same exception when i have created firebase-function using firebase-tool

const ref = admin.database().ref("path/to/database/object");

ref.once("value").catch(error =>{  // line 22
    response.send( error() );
}).then( snapshot =>{
    response.send( snapshot.val );
})

This code doesn not compiled and return

ERROR: /src/index.ts[22, 5]: Promises must be handled appropriately

I have changed the places of catch and then.

ref.once(...).then(...).catch(...)

This code is work, i am sorry but I don't have any explanation

So much amazing when app return the some error without catch block even according firebase doc not mentioned that catch is required.

Answer 4

Your getStatus function is defined to return a promise:

// All functions marked as async returns a promise:
async function getStatus(message: Message) {/* ... */}

But you called getStatus without calling it's then:

getStatus(message)

Therefore the compiler thinks you've forgotten to handle your async code. All you need to do is call .then():

getStatus(message).then(() => console.log('done'));

Answer 5

If you are writing a void async function with typescript-eslint, you will be getting a complaint if you write async functions without await, it's easy to forget especially for the last line of the function.

const sendSyncPlayCommandToWatchers = async (): Promise<void> => {
  ...
  someAsyncFunction()
}

You will get

error Promises must be handled appropriately @typescript-eslint/no-floating-promises

You need to add the await keyword to the last line (or make sure you handle the promise with try/catch as mentioned in previous replies

const sendSyncPlayCommandToWatchers = async () => {
  ...
  await someAsyncFunction()
}

I think this will also help getting a better stack trace when debugging.

Answer 6

I think this problem is fixable by awaiting the getStatus function, since its an async function. The message is saying it clearly, but the line number causes confusion. To be honest it also took me some time.

You can solve this lint error by this change in code:

client.on('message', message => {
 if (message.content === 'green') {
   await getStatus(message)
}});

In my opinion it's not a good idea to switching these specific errors off. They are useful because in the other way you would not run the code async.