discriminated union type narrow when destructuring object within desturct array

[wusiquan]

🔎 Search Terms

destructuring object within destructured array, discriminated union

🕗 Version & Regression Information

the error part appeared in every version I tried

⏯ Playground Link

https://www.typescriptlang.org/play?target=1&ts=5.2.2#code/C4TwDgpgBAYglgJwM7AJLAgWygXigbwCgopRIAuKAcgDNEUrioA3AQwBsBXCSlBOAHYBzQgF9ChMtADKEAMYB7AQBN0WXASZTKVJPKXLGJNlx5QBnTACMICMRKlQAKgAtEqjNjxES26sDcEQyYTbkorBQV2CFYBe0JFARQoVkoACnhkNE8oAB8oWUSPdXzXdzVMAEoAbQBdDTqJAHom0ghkgEZCVgA6GgUEAFFWORc0tPxScAgAGhYObihRStwAPk0SOBooNMccfeo6LKoVnxIoFqgFAGteYH5hJhJEpCiIHvYFITTQiEqmcTLZqtDDJABMhH6CB2L2ABCmkDmvyWV22rFOTC2Oz2B1o9GAJw250uNzuDxE5ygLzeHy+PwWfwB8QSSmSVko1Vg+IqeQK+hUPNKgWK2FqdQatWBbWSAGZIQMYay4ZzmFB6gptlYMZttj8eji8Hjjtria0bk8qayaZ9vswer9-iRAVLQXCACyEKx9AbDUbjaqTKRIhlLWorHDrM5Y3bTXC4o4ME0kS4CBRw0lQPiCIS8izWWy8iJvWIW6nRWm2hmOpZif6ES6uqAAVnl0LSsKgnMD02DphR6s1JujBsO+MJZ2TrVT6dumfu2dzlhs0PyReiJcpZfeNvppmrgKAA

💻 Code

type FirstItem = {
  type: 'first'
  value: string
}

type SecondItem = {
  type: 'second'
  value: number
}

type ThirdItem = {
  type: 'third'
  value: boolean
}

const a: (FirstItem | SecondItem | ThirdItem)[] = []

// test 1
a.forEach(({ type, value }) => {
  if (type === 'first') {
    // ok: string
    console.log(value)
  }
})

// test 2
for (const { type, value } of a) {
  if (type === 'first') {
    // ok: string
    console.log(value)
  }
}


const b: [ FirstItem | SecondItem | ThirdItem ][] = []

// test 3
for (const [ v ] of b) {
  if (v.type === 'first') {
    // ok
    console.log(v.value)
  }
}

// test 4
b.forEach(([{ type, value }]) => {
  if (type === 'first') {
    // not ok: string | number | boolean
    console.log(value)
  }
})

// test 5
for (const [ { type, value } ] of b) {
  if (type === 'first') {
    // not ok: string | number | boolean
    console.log(value)
  }
}

🙁 Actual behavior

the code above, test4 and test5 can not narrow to the string type like test1, test2 or test3

🙂 Expected behavior

I expected the value in the loop to infer successfully

Additional information about the issue

No response

[jcalz]

Please edit the title to use the term "destructuring" instead of "desturcting" so that title searches can find it.

---

Looks like a missing feature as opposed to a bug, since #46266 only operates if the thing being destructured is a discriminated union, and [ FirstItem | SecondItem | ThirdItem ][] is not a discriminated union.

#46266 (comment)

[wusiquan]

@jcalz thank you. I agree this is more than a missing feature, after reading the linked issue. Type [ FirstItem | SecondItem | ThirdItem ], the first item of the array type, is a discriminated union, but it is nested. Destructuring the nested object(array) all at once or in a seperate way, when using forEach and for...of, the results are not the same. It seems a little complex.

[kykungz]

Related use case: TanStack/query#6162

[mattfbacon]

Is this the same issue?

// gen: Generator<number>
let value;
while (!({ value } = gen.next()).done) {
  // `value` is not properly inferred as number here.
}