Errors get swallowed when nested selectors are used

[jspri]

Subject of the issue

Errors that are thrown within the xray callback are suppressed when there is a nested selector.

Your environment

• version of node: 6.3.1
• version of npm: 3.9.1

  Reproducing

The following script has no output when run. You would expect an error message/stack trace to appear.

const Xray = require('x-ray')
const x = Xray()

const map = {
    title: 'title',
    links: x('a', ['@href'])
}

x('http://www.google.com', map)(function(err, res) {
    throw(Error("this is an error"))
})

[jspri]

A bit more investigation show that if the code is changed to the following the error will be shown. This suggests that the callback is being called twice, once to handle the result set and once to handle the error.

x('http://www.google.com', map)(function(err, res) {
    if(err) {
        console.log(err)
    }

    throw(Error("this is an error"))
})

Alternatively if a user were to do something like this...

function scrapeSomething(callback) {
    x('http://www.google.com', map)(function(err, res) {
        if(err) {
            callback(err)
        }

        throw(Error("this is an error"))
    })
}

The more idiomatic node code in the second example would result in the error bubbling up to closer match what the expected behaviour would be. Likely making the bug less likely to surface in many situations.

[jspri]

Possibly related to #135/#181. Reopened as x-ray 2.3.1 did not fix.

[jspri]

Found to be issue with batch library https://github.com/visionmedia/batch/issues/32