try/catch with Types
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Basic try/catch
A try block contains code that may fail, while the catch block handles whatever exception gets thrown — in TypeScript, the caught value is typed as unknown by default rather than any, which forces you to check it before use.
Example: Basic try/catch
try {
throw new Error("Something broke");
} catch (err: unknown) {
console.log(typeof err);
}Narrowing an Error
An unknown caught value should be narrowed — with instanceof Error, for example — before accessing properties like message, since TypeScript won't let you read properties off an unknown value without first proving what shape it has.
Example: Narrowing an Error
try {
throw new Error("Failed");
} catch (err: unknown) {
if (err instanceof Error) {
console.log(err.message);
}
}Handling Unknown Thrown Values
JavaScript allows literally anything to be thrown, not just Error objects, so TypeScript code should never assume a caught value is an Error without checking — a thrown string or plain object is completely legal JavaScript.
Example: Handling Unknown Thrown Values
try {
throw "just a string";
} catch (err: unknown) {
if (typeof err === "string") {
console.log("String thrown:", err);
} else if (err instanceof Error) {
console.log("Error thrown:", err.message);
}
}Typed Error Handling Helpers
A small helper function can centralize the logic for turning any caught value into a readable message, keeping the narrowing boilerplate in one place instead of repeated at every catch block in the app.
Example: Typed Error Handling Helpers
function toMessage(err: unknown): string {
if (err instanceof Error) return err.message;
if (typeof err === "string") return err;
return "Unknown error";
}
try {
throw new Error("Oops");
} catch (err) {
console.log(toMessage(err));
}Best Practices
Treat caught values as unknown, narrow them before use, and always provide a sensible fallback message for the case where the thrown value isn't a standard Error — this keeps error handling predictable even with badly-behaved third-party code.
Example: Best Practices
function toMessage(err: unknown): string {
return err instanceof Error ? err.message : "Something went wrong";
}
try {
throw { weird: "object" };
} catch (err) {
console.log(toMessage(err));
}Chapter Quiz — Complete all 5 topics to unlock
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