types-not-correct-in-with-callback.md

name	about	title	labels	assignees
Types Not Correct in/with Callback	TypeScript assuming the wrong type either after a callback runs, or within a callback		Duplicate

TypeScript has two narrowing-related behaviors that are both intentional. Please do not log additional bugs on this; see #9998 for more discussion.

The first is that narrowings are not respected in callbacks. In other words:

function fn(obj: { name: string | number }) {
  if (typeof obj.name === "string") {
    // Errors
    window.setTimeout(() => console.log(obj.name.toLowerCase());
  }
}

This is intentional since the value of obj.name "could" change types between when the narrowing occurred and when the callback was invoke. See also #11498

The second is that function calls do not reset narrowings. In other words:

function fn(obj: { name: string | number }) {
  if (typeof obj.name === "string") {
    console.log("Here");
    // Does not error
    console.log(obj.name.toLowerCase());
  }
}

This is intentional behavior, even though console.log could have mutated obj. This rule is consistently applied, even with the function is in-principle inspectable to actually have side effects

function fn(obj: { name: string | number }) {
  if (typeof obj.name === "string") {
    mut();
    // Does not error
    console.log(obj.name.toLowerCase());
  }

  function mut() {
    obj.name = 42;
  }
}