meet it

Return a result so callers cannot forget the failure.

Some calls fail: JSON.parse throws on broken text. runs the try block and, if anything throws, jumps to the catch block instead of crashing. The error it catches is unknown, because anything can be thrown, so check it before you use it.

A thrown error is invisible in a function's type, so callers forget it. A makes failure part of the return type: a discriminated union of { ok: true; value: T } and { ok: false; error: string }. Callers must check ok before they can reach the value.

type Result<T> = { ok: true; value: T } | { ok: false; error: string };
function parseJson(text: string): Result<unknown> {
  try {
    return { ok: true, value: JSON.parse(text) };
  } catch {
    return { ok: false, error: `Not valid JSON: ${text}` };
  }
}
console.log(parseJson('{"cups": 2}'));
console.log(parseJson("{cups: 2}"));

If you do not need the error, write catch with no brackets after it.

guess first

Share the sweets. What happens?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

type Result<T> = { ok: true; value: T } | { ok: false; error: string };
function share(sweets: number, people: number): Result<number> {
  if (people === 0) {
    return { ok: false, error: "Nobody to share with" };
  }
  return { ok: true, value: sweets / people };
}
const each = share(12, 3);
console.log(each.value);

your turn

Your turn. Turn an answer into an age.

Fill in ok for the failure and for the success, then type an age as text.

Your code

press run to see what happens

fix it

The JSON checker will not build. Fix the catch.

Broken code to fix

press run to see what happens

Read the error: TS18046 says e is of type unknown.
Anything can be thrown, even a number, so e might have no name. Narrow it first.
Wrap the return in if (e instanceof Error) { ... } and return "invalid" after it.

nailed it

your failures are part of the plan now. :D

try...catch turns a crash into code you control.
The caught error is unknown, so narrow it before use.
A Result type makes callers check ok before the value.

new words in your collection

try...catch
Result type

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