Null checks returned from function fails to return accurate types

[livingforjesus]

🔎 Search Terms

"null checking", "function inference", "null check inference", "function null check"

🕗 Version & Regression Information

• This is a crash
• This changed between versions ______ and _______
• This changed in commit or PR _______
• This is the behavior in every version I tried, and I reviewed the FAQ for entries about function inference, null check
• I was unable to test this on prior versions because _______

⏯ Playground Link

https://www.typescriptlang.org/play/?#code/JYOwLgpgTgZghgYwgAgLIE8CS5rycgbwChllgATALmQGcwpQBzE5EAVwBsO4AjDianQYhGyAD6tOHIgF8iRGGxAIwwAPYhk8YP3LYY0CMogAKAJSEWCDXTIgADmzDUM2SLEQoAvJdKkK1ADkFIEANCyk7Fy8-EEgGgC0URyBLHL+MMgmAISgjmAAdMkxEBbEfshgABZQagDurBANAKJQtVDmafKkUBBgbFCaeU6y8tYgtpB0AGJwOhB6IAa9xsg+2rr6hsadAPS7yAAqVcA0tFVqnOQggWBacxzIPBAIcGw0KNUoisqqGsgANzgHAocCmlSqYIhKGKfG+wAgHHIZDO8RASSkJSeTjIdw2NCI-DusP4ABkjIxqmtKhAZg8FlsVkgipi4QV+CJqkA

💻 Code

// Your code here
interface MyInterface {
  id: string
  nullable: string | null
}

function failedInference() {
  const input: MyInterface = {
    id: 'id',
    nullable: 'non-null'
  }
  if (!input.nullable) {
    throw new Error()
  }

  return input
}

const testFailedInference = failedInference()
// This shouldn't fail because the function validates that the nullable field is non-nullable but it fails
let nullableLength = testFailedInference.nullable.length

🙁 Actual behavior

The type of the returned testFailedInference.nullable is still string | null and this is wrong because I already validated it to be non nullable in the function and typescript is inferring function return type

🙂 Expected behavior

The type of the returned testFailedInference.nullable is just string and not nullable.

Additional information about the issue

Implementing it in a form-factor like this works

function successInference() {
  const input: MyInterface = {
    id: 'id',
    nullable: 'non-null'
  }
  if (!input.nullable) {
    throw new Error()
  }

  return {
    ...input,
    nullable: input.nullable
  }
}
const testSuccessInference = successInference()
// This succeeds after manually specifying in the function return
let nullable2Length = testSuccessInference.nullable.length

[MartinJohns]

This is working as intended. Checks within functions do not propagate to outside the function, unless you're writing an explicit type guard or assertion function. Your function returns a MyInterface, and the nullable property can be null.

See also #42384.

[typescript-bot]

This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.