TypeScript · Junior · Question 57 of 75

How does `instanceof` help with types?

if (err instanceof HttpError) {
  console.log(err.status);
}

✓Correct answer: It narrows the value to the class type inside the branch
2It converts the value to the class
3It works only with interfaces

Explanation

instanceof checks the prototype chain at runtime, so it works with classes, not with interfaces or type aliases (they don't exist at runtime).

Further reading

TS Handbook: Narrowing ↗

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