Errors

try, catch, finally and throw work as in TypeScript. throw takes a new Error(…), a RangeError, a TypeError, a class that extends one of them, or what a catch took (throw e). A class of error has a message, a name and fields of its own, and e instanceof OutOfGold tells the classes apart.

class OutOfGold extends Error {
  constructor(public need: number) { super(`need ${need} more`); }
}

program(() => {
  let gold = 30;
  function buy(cost: number) {
    if (cost > gold) throw new OutOfGold(cost - gold);
    gold -= cost;
  }
  try {
    buy(20);
    buy(50);
  } catch (e) {
    if (e instanceof OutOfGold) print(`${e.message} (${e.need})`);   // need 40 more (40)
    else throw e;
  } finally {
    print(`${gold} left`);                      // 10 left
  }
});

• An error goes up through the functions to the nearest catch. finally runs however the try is left: a return, a break, or an error on its way out.
• An error nothing catches is shown in the game in red, with its line ("TrigScript: uncaught OutOfGold: need 20 more, line 12."), and Simulate lists it at the throw. The program or task stops there. A handler of an event gives up that one turn and goes on listening.
• A try may span sleep and await. An error in one branch of Promise.all or Promise.race leaves the statement, and the other branches stop where they are.
• A caught error cannot be kept in a variable, an array or a field. Copy what you need out of it in the catch, or put it in a field of your own class.
• An index past an array's end and a full pool are not errors: the game passes over them, and Simulate and the tests say where.