How to declare a function that throws an error in Typescript

Question

In Java I would declare a function like this:

public boolean Test(boolean test) throws Exception {
  if (test == true)
    return false;
  throw new Excepti         


        

Answer 1

You cannot using pure ts (v<3.9) I hope it will be available in the future. A workaround is however possible, it consists of hiding the possible thrown types in the method's signature to then recover those types in the catch block. I made a package with this workaround here: https://www.npmjs.com/package/ts-throwable/v/latest

usage is more or less as follow:

import { throwable, getTypedError } from 'ts-throwable';
class CustomError extends Error { /*...*/ }

function brokenMethod(): number & throwable {
    if (Math.random() < 0.5) { return 42 };
    throw new CustomError("Boom!");
}

try {
    const answer: number = brokenMethod()
}
catch(error){
    // `typedError` is now an alias of `error` and typed as `CustomError` 
    const typedError = getTypedError(error, brokenMethod);
}