New rule or no-floating-promises upgrade to prevent parallel pending promises before await

[ackvf]

RFC

Given a new rule or no-floating-promises rule, when an await is encountered AND there are more than one pending promises, the rule will suggest to use await Promise.allSettled or to append .catch() to prevent unexpected behavior that may result in uncaught errors.

Additional Info

(Migrated from eslint/eslint#17824 (comment))

In a situation like this, when a promise is awaited (and later fails), while another has already failed, it results in an uncaught error.
In the example below, the failed promise is at the line const r2 = await p2, when it throws, the following code never runs and therefore the remaining promises are never awaited.

More explanation https://stackoverflow.com/a/69644993/985454

try {
  const p1 = Promise.resolve('p1:success')
  const p2 = Promise.reject('p2:failure')
  const p3 = new Promise((resolve, _) => {
    setTimeout(() => {
      console.log('p3 finished')
      resolve("p3:success")
    }, 3000)
  })
  const p4 = new Promise((_, reject) => {
    setTimeout(() => {
      console.log('p4 finished')
      reject("p4:failure")
    }, 1000)
  })
  

  const r1 = await p1
  const r2 = await p2 // ❌following code never runs!
  const r3 = await p3
  const r4 = await p4
  console.log('await:', [r1, r2, r3, r4])


  // this would also throw ❌ - ✅ caught by no-floating-promises rule
  // Promise.all([p1, p2, p3, p4]).then((values) => {
  //   console.log('all:', values)
  // })


  // this would not throw ✅
  // await Promise.all([p1, p2, p3, p4]).then((values) => {
  //   console.log('all:', values)
  // })


} catch (e) {
  console.log('catch:', e)
}

Strangely enough, the no-floating-promises rule does not catch these cases

const p2 = Promise.reject('p2:failure') // ❌ no error given

const p4 = new Promise((_, reject) => { // ❌ no error given
  setTimeout(() => {
    console.log('p4 finished')
    reject("p4:failure")
  }, 1000)
})

Promise.all([p1, p2, p3, p4]).then((values) => { // ✅ error: Promises must be awaited, end with a call to .catch, ...
  console.log('all:', values)
})

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[bradzacher]

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