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✅ Viability Checklist

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This isn't a request to add a new utility type: https://github.com/microsoft/TypeScript/wiki/No-New-Utility-Types
• This feature would agree with the rest of our Design Goals: https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals

⭐ Suggestion

Current syntax is an error in TS. Support a syntax which is interpreted as a throw type annotation in ErrorScript but ignored in TS. For example:

// Definition for TS, ignored by ErrorScript
namespace ErrorScript {
  type Result<T, E> = T // This type is valid only as a return type
  type Promise<T, E> = Promise<T>
}

function maybeGetNumber(): ErrorScript.Result<number, DOMException> {
  const n = Math.random()
  if (n < 0.5) throw new DOMException("Too low", "RandomError")
  return n
}

async function maybeGetNumberLater(): ErrorScript.Promise<number, DOMException> {
  const n = Math.random()
  if (n < 0.5) throw new DOMException("Too low", "RandomError")
  return n
}

Additionally, for JS with JSDoc, a @throws tag could be used:

/**
 * @returns {number}
 * @throws {DOMException}
 */
function maybeGetNumber() {
  const n = Math.random()
  if (n < 0.5) throw new DOMException("Too low", "RandomError")
  return n
}

Also it would be worth adding a JS mode to the playground so the JSDoc support can be tested too.

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💻 Use Cases

Easier migration. Not everybody has to switch at once.