try...catch...finally

try/catch handles thrown exceptions and rejected awaited Promises in async functions. finally executes when control leaves the try/catch path, whether successful or failed. It is useful for releasing resources or resetting UI loading state.

Reading the code: The Error is thrown, then the catch block narrows unknown to Error and prints unavailable. finally runs after the catch and prints cleanup.

Keep the boundary clear: finally does not magically undo side effects. A return inside finally can override an earlier return or thrown error, so avoid that pattern.

Save the code as try-catch-finally.ts. The result below was captured after compiling this exact example with strict TypeScript checking and executing the emitted JavaScript in Node.js.

TypeScript code

async function main(): Promise<void> {
  try {
    throw new Error("unavailable");
  } catch (error: unknown) {
    if (error instanceof Error) console.log(error.message);
  } finally {
    console.log("cleanup");
  }
}
main().catch(console.error);

Verified output

unavailable
cleanup

Build: npx tsc --strict --target es2022 --module commonjs --lib es2022,dom,esnext.disposable try-catch-finally.ts; run: node try-catch-finally.js. The exact filename may be changed freely. When a multi-file project exists, prefer its tsconfig.json and build scripts.

The Error is thrown, then the catch block narrows unknown to Error and prints unavailable. finally runs after the catch and prints cleanup.

The output shown above comes from the initialized values in this program. Replacing a type annotation alone does not transform a JavaScript value.

finally does not magically undo side effects. A return inside finally can override an earlier return or thrown error, so avoid that pattern.

Check both the TypeScript diagnostic and the emitted JavaScript behavior. A program can compile correctly and still need runtime validation or error handling.

Replace throw with await Promise.reject(new Error("failed")) and predict the same cleanup order.

Before running your change, write down the type you expect TypeScript to infer and the output you expect JavaScript to print. Then check both predictions.

try/catch handles thrown exceptions and rejected awaited Promises in async functions.

Try the practice change and re-run the exact program to test your understanding.