Callbacks called twice when containing errors

[jspri]

When the callback provided to batch.end throws an error things can go a bit haywire... Here is an example I put together.

const Batch = require('batch')
const batch = new Batch()

batch.push(cb => cb())
batch.push(cb => cb())
batch.push(cb => cb())

var i = 1;

batch.end(function(err) {
  console.log(i++)

  if(err) {
    console.log(err)
  }

  throw "Error"
})

Outputs

1
2
Error

This shows the callback being called twice, once with no error (expected) and once with the error contained in the callback! After doing a bit of digging I found the try/catch block in index.js:117 to be the culprit. I assume this was put in so that synchronous type functions would produce callback style errors. It's a really messy bug to have though because it can catch all downstream errors of the callback.