JavaScript での Go スタイルのエラー処理の解釈

Almost everyone who uses JavaScript daily knows that try-catch can be painful to deal with, especially when you have more than one error to handle.

Most proposed solutions are tryng to copy Golang's approach - which handles everything as return values. It is, among other things, is a great feature of Go, but JS is completely different language (duh) and I think we can do better than a copy-paste from Go.

In Go, when we want to handle an error, we return it from the function call either as a second value in tuple or as return value of the function call. Following is the pattern:

result, error := DoSomething()
if error != nil {
 // handle error
}

This approach allows to handle the errors explicitly using standard control flow.

To apply this pattern in javascript the most common solution is to return results as an array:

const handler = async (promise) => {
  try {
    const result = await promise()
    return [result, null];
  } catch(error) {
    return [null, error];
  }
}

const [response, error] = await handle(fetch('http://go.gl'))
if (error !== null) {
  // handle error
}

As you can see this is almost direct copy-paste of the pattern from Go.

Returning consistent value

This pattern works great, but in javascript we can do better than this. The core idea of this pattern is to return error as a value, so let's adapt it with better SoC.

Instead of returning null or Error we can decorate the result with a consistent interface. That would improve our SoC, and give us a strongly typed return value:

interface Status {
  Ok(): boolean;
  Fail(): boolean;
  Of(cls: any): boolean;
}

The interface Status doesn't have to be an Error, but we can check it's type using status.Of(Error). We can always return an object that sattisfies Status. The usage example would be:

const [response, error] = await handle(res.json())
if (error.Of(SyntaxError)) {
  // handle error
  console.log("not a json")
  return
}

Now, in JavaScript our result doesn't always have to be a tuple. We can actually create our own class that behaves as a tuple when it's needed:

interface IResult<t> {
  0: T;
  1: Status;
  value: T;
  status: Status;
  Of(cls: any): boolean;
  Ok(): boolean;
  Fail(): boolean;
}
</t>

Usage example:

const result = await handle(res.value.json())
if (result.Of(SyntaxError)) {
  // handle error
  console.log("not a json")
  return
}

The Implementation

Following this approach I've created ready to use function - Grip.

Grip is strongly typed and can decorate functions and promisises alike.

I use git to host such packages, so to install use github:

bun add github:nesterow/grip # or pnpm

Usage:

The grip function accepts a function or a promise and returns a result with return value and status.
The result can be hadled as either an object or a tuple.

import { grip } from '@nesterow/grip';

Handle result as an object:

The result can be handled as an object: {value, status, Ok(), Fail(), Of(type)}

const res = await grip(
  fetch('https://api.example.com')
);

if (res.Fail()) {
    handleErrorProperly();
    return;
}

const json = await grip(
  res.value.json()
);

if (json.Of(SyntaxError)) {
    handleJsonParseError();
    return;
}

Handle result as a tuple:

The result can also be received as a tuple if you want to handle errors in Go'ish style:

const [res, fetchStatus] = await grip(
  fetch('https://api.example.com')
);
if (fetchStatus.Fail()) {
    handleErrorProperly();
    return;
}

const [json, parseStatus] = await grip(
  res.json()
);
if (parseStatus.Of(SyntaxError)) {
    handleJsonParseError();
    return;
}

If you like this take on error handling check out the repository. The source is about 50LOC, without types, and a 100 with types.

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