Difficult to determine that an error is the result of bad JSON

Having SyntaxErrors consistently returned (#119) and a status code provided means we can fairly reliably detect errors in express middleware that are the result of bad JSON.

app.use(function(err, req, res, next) {
  if (err instanceof SyntaxError && err.status === 400) {
    console.error('Bad JSON');
  }
});

This test could theoretically give false positives though, if some other middleware were to throw something similar. Imagine we’re using body-parser first and then something else to validate the body has the structure we’re expecting, for example.

Is there any way to detect JSON parsing error with 100% reliability with the library as it is now? Is there something you could (perhaps in 2.0) to make this a little easier?

[dougwilson]

All errors include a type property from 1.18.0 release onwards. For parse failure, err.type === 'entity.parse.failed'.

[carlbennettnz]

That’s fine, I trust your judgement 😃

I’ll stick with

app.use(function(err, req, res, next) {
  if (err instanceof SyntaxError && err.status === 400 && 'body' in err) {
    console.error('Bad JSON');
  }
});