Prefer return objects instead of exceptions

June 25, 2016

I have been toying with several other languages lately like Rust and Go. The thing I love about these languages is a lack of exceptions. Things are easier to reason about and no try/catch. It seems to me, with generators and async/await, that try/catch is turning into the preferred error handling style for JavaScript. I said as much in response to a comment when I spoke at the Seattle Node.js meetup. The room collectively groaned, and internally I groaned with them. However, it is hard to deny the way async stuff is currently shaping up in the specs.

I feel like there is a way to improve this. Go uses tuples and Rust uses Results/Options structs as their standard error handling techniques. I think we can combine the two ideas in JS using some of the newer features of ES2015 and ES2016 for an elegant way of writing code.

The Foundation

The underlying thing for this to work is for every function to return a special standard object with a set of result and error properties. An example function might look like this:

const exampleFunction = function(something) { let resultValue, errorObject; if(!something) { errorObject = new Error("An error has occurred."); } else { resultValue = something + something; } return { result: resultValue, //required if error null or undefined error: errorObject //required if result null or undefined }; };

What about existing code that still uses Exceptions?

One thing that throws (pun intended) a monkey wrench into things are existing modules that use exceptions. For that I propose a very simple wrapper method that might look like this:

const wrap = function (functionToWrap, argsArray = []) {
  try {
    return { result: functionToWrap(...argsArray) }
  } catch (e) {
    return { error: e }
  }
}

And Async?

If you want to do something similar with promises you might have this function:

const wrapAsync = async function (functionToWrap, argsArray = []) {
  try {
    return { result: await functionToWrap(...argsArray) }
  } catch (e) {
    return { error: e }
  }
}

Final examples

When you put it all together you get code that might look like this:

;(function () {
  const { result, error } = wrap(exampleFunction, ["testing"])
  if (error) {
    //do something with the error
  } else {
    //do something with the result
  }
})()
;(async function () {
  const { result, error } = await wrapAsync(exampleAsyncFunction, ["testing"])
  const { result2, error2 } = await wrapAsync(exampleAsyncFunction, ["testing"])
  if (error || error2) {
    //do something with the error
  } else {
    //do something with the result
  }
})()

Realistically, there is a performance hit for using many try/catch statements instead of a big one at the top level. Wrapping everything like this could be impactful. Try/catch also keeps the compiler from optimizing the wrapped code, however the code inside of functions you are wrapping can still be optimized, so I think that is ok.

The only way to know the performance impact is to test it in a real (albeit small and simple) api or app, which is just what I will do. I will update this post with results when I have them.

Chris Griffing