Error Cause

Status: Stage 4

Author: @legendecas

Champion: @legendecas, @hemanth

Chaining Errors

Errors will be constructed to represent runtime abnormalities. To help unexpected behavior diagnosis, errors need to be augmented with contextual information like error messages, error instance properties to explain what happened at the time.

If the error were thrown from deep internal methods, the thrown error may not be straightforward to be easily conducted without proper exception design pattern. Catching an error and throwing it with additional contextual data is a common approach of error handling pattern. Multiple methods are available to augment the caught error with additional contextual information:

async function doJob() {
  const rawResource = await fetch('//domain/resource-a')
    .catch(err => {
      // How to wrap the error properly?
      // 1. throw new Error('Download raw resource failed: ' + err.message);
      // 2. const wrapErr = new Error('Download raw resource failed');
      //    wrapErr.cause = err;
      //    throw wrapErr;
      // 3. class CustomError extends Error {
      //      constructor(msg, cause) {
      //        super(msg);
      //        this.cause = cause;
      //      }
      //    }
      //    throw new CustomError('Download raw resource failed', err);
    })
  const jobResult = doComputationalHeavyJob(rawResource);
  await fetch('//domain/upload', { method: 'POST', body: jobResult });
}

await doJob(); // => TypeError: Failed to fetch

If the errors were chained with causes, it can be greatly helpful to diagnosing unexpected exceptions. As the example above shows, quite a few laboring works has to be done for a simple error handling case to augmenting the caught error with contextual message. Besides, no consensus on which property will representing the cause makes it not feasible for developer tools to revealing contextual information of causes.

The proposed solution is adding an additional options parameter to the Error() constructor with a cause property, the value of which will be assigned to the error instances as a property. So errors can be chained without unnecessary and overelaborate formalities on wrapping the errors in conditions.

async function doJob() {
  const rawResource = await fetch('//domain/resource-a')
    .catch(err => {
      throw new Error('Download raw resource failed', { cause: err });
    });
  const jobResult = doComputationalHeavyJob(rawResource);
  await fetch('//domain/upload', { method: 'POST', body: jobResult })
    .catch(err => {
      throw new Error('Upload job result failed', { cause: err });
    });
}

try {
  await doJob();
} catch (e) {
  console.log(e);
  console.log('Caused by', e.cause);
}
// Error: Upload job result failed
// Caused by TypeError: Failed to fetch

Compatibilities

In Firefox, Error() constructor can receive two optional additional positional parameters: fileName, lineNumber. Those parameters will be assigned to newly constructed error instances with the name fileName and lineNumber respectively.

However, no standard on either ECMAScript or Web were defined on such behavior. Since the second parameter under this proposal must be an object with a cause property, it will be distinguishable from a string.

Implementations

Polyfills:

error-cause: An es-shim interface polyfill.
Pony Cause: Helpers to work with ECMAScript error cause and VError style error cause.

JavaScript Environments:

Chrome, released on 93,
Firefox, released on 91,
Safari, released on 15,
Node.js, released on v16.9.0.

FAQs

Differences with `AggregateError`

The key difference between those two is that the errors in the AggregateError doesn't have to be related. AggregateError are just a bunch of errors just happened to be caught and aggregated in one place, they can be totally unrelated. E.g. jobA and jobB can do nothing to each other in Promise.allSettled([ jobA, jobB ]). However, if an error were thrown from several levels deep of jobA, the cause property can accumulate context information of those levels to help understanding what happened exactly.

With AggregateError, we get breadth. With the cause property, we get depth.

Why not custom subclasses of `Error`

While there are lots of ways to achieve the behavior of the proposal, if the cause property is explicitly defined by the language, debug tooling can reliably use this info rather than contracting with developers to construct an error properly.

License

tc39/proposal-error-cause

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