try / catch Error Handling & catch Narrowing

Last updated: 2026-10-22

try / catch Error Handling & catch Narrowing

TypeScript error handling has two pain points: the catch variable is unknown under strict mode (JS can throw anything, so the compiler dare not assume Error), requiring narrowing before you can touch .message; and "this may throw" is invisible in type signatures. The fixes: narrow unknown with instanceof, and for expected failures use the Result pattern, encoding success/failure right in the return type.

ℹ️ Info The editor transpiles TypeScript to JavaScript and runs it (no type checking); see each example's Output for type-check results.

📝 Syntax

// 1) Narrowing unknown
try {
  risky();
} catch (err: unknown) {
  if (err instanceof Error) console.log(err.message);
  else console.log("unknown throw:", err);
}

// 2) Result pattern: a union type encoding success or failure
type Result<T> = { ok: true; value: T } | { ok: false; error: string };

function attempt<T>(fn: () => T): Result<T> {
  try { return { ok: true, value: fn() }; }
  catch { return { ok: false, error: "failed" }; }
}

⚙️ Parameters

Returns：The Result pattern's return type is a discriminated union: value exists only when ok is true, error only when false — possible failure is encoded in the type, and callers cannot compile without handling the failure branch.

▶ Example

Edit the code below and click Try it to see the transpiled result:

Output：

caught RangeError: not a positive integer: abc
7 0
{"ok":true,"value":42}
failed: invalid input

(Runtime output after transpilation; .message is only reachable after instanceof narrowing — touching err.message without it yields error TS18046)

💡 Notes

🏷️ Related Engineering & Practice

💡 Related cases

📚 Related tutorials

❓ FAQ

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