TypeScript · Senior · Question 41 of 75

Is the narrowing of a `let` variable preserved inside a callback?

function f(x: string | number) {
  if (typeof x === 'string') {
    setTimeout(() => x.toUpperCase());
  }
}

✓Correct answer: Yes, since TS 5.4, if the variable isn't reassigned afterwards
2No, never: narrowing is always reset in closures
3Yes, but only for const variables in any version

Explanation

Previously, inside a callback TS assumed x might be reassigned and reset the narrowing to string | number. Since 5.4, the compiler understands the value is stable after the last assignment and keeps the narrowing. For const it already worked before.

Is the narrowing of a `let` variable preserved inside a callback?

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Is the narrowing of a let variable preserved inside a callback?

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Is the narrowing of a `let` variable preserved inside a callback?