instrument.js

var xhr       = require('xhr')
var shoe      = require('shoe')('/smokestack')
var slice     = require('sliced')
var isDom     = require('is-dom')
var format    = require('util').format
var convert   = require('convert-source-map')
var console   = window.console
var SourceMap = require('source-map').SourceMapConsumer
var mapErrors = require('source-map-support')
var cachedSourceMap
var cachedBody

// keep around so can call
// console methods without sending data to server
var nativeConsole = {}

;['error'
, 'info'
, 'log'
, 'debug'
, 'warn'
].forEach(function(k) {
  var nativeMethod = console[k]
  nativeConsole[k] = nativeMethod.bind(console)
  var prefix = k

  console[k] = function() {
    // keep original args so browser can log as usual
    var args = slice(arguments)
    write(prefix, args)
    return nativeMethod.apply(this, args)
  }
})

function write(prefix, args) {
  // prepare args for transport
  var cleanArgs = args.map(function(item) {
    // no sensible default for stringifying
    // DOM Elements nicely so just toString and let
    // whoever is logging handle stringification.
    if (item && isDom(item)) return item.toString()
    return item
  })

  var output = format.apply(null, cleanArgs)
  var data = JSON.stringify([prefix].concat(output))

  shoe.write(data)
  shoe.write('\n')
}

var close = window.close

window.close = function() {
  setTimeout(function() {
    shoe.write(JSON.stringify({ end: true }))
    shoe.write('\n')
    shoe.once('data', function() {
      shoe.end()
    })
  })
}

shoe.on('end', function() {
  close()
})

xhr('script.js', function(err, resp) {
  if (err) return console.error(err)

  // large sourcemaps will fail to parse, this is suprisingly common.
  // so, we'll use the "large source map option", which is faster anyway
  cachedBody = resp.body || ''
  cachedSourceMap = convert.fromSource(cachedBody, true)

  // magical auto-correction of error stack traces in v8
  if (!cachedSourceMap) {
    mapErrors.install({handleUncaughtExceptions: false})
  } else {
    mapErrors.install({
        handleUncaughtExceptions: false
      , retrieveSourceMap: function(source) {
        return {
            url: source
          , map: cachedSourceMap && cachedSourceMap.sourcemap || ''
        }
      }
    })
  }

  start()
})

window.onerror = function(message, filename, line, col, error) {
  var supportsError = !!error
  if (!supportsError) {
    var error = new Error(message)
    error.stack = 'Error: '+message+'\n    at '+filename+':'+line+':'+col
  }
  error.fileName = error.fileName || filename
  error.lineNumber = error.lineNumber || line
  error.columnNumber = error.columnNumber || col
  if (supportsError && cachedSourceMap) {
    var sm = cachedSourceMap.toObject()
    var smc = new SourceMap(sm)
    var original = smc.originalPositionFor({line: error.lineNumber, column: error.columnNumber})
    var source = smc.sourceContentFor(original.source)
    logError(source, error, original)
  }
  else {
    logError(cachedBody, error, {
      source: filename
      , line: line
      , column: col
    })
  }
  window.close()
}

function logError(src, error, position) {
  var lines = src.trim().split('\n')
  position = position || {
    line: error.lineNumber,
    column: error.columnNumber,
    source: error.filename
  }
  var errorLine = '' + lines[position.line - 1]
  var spaces = ''

  // write in tap format so that test fail when there's an uncaught exception
  write('error', ['not ok - ' + error.message])

  write('error', [position.source + ':' + position.line])

  for (var i = 0; i < position.column; i++) {
    spaces += ' '
  }
  var columnLine = spaces + '^'
  var errorLines = [errorLine, columnLine]
  // write different data to remote console
  // and window console.
  write('error', [errorLines.join('\n') + '\n'])

  // it's possible that we can fail to get a stack
  try {
    write('error', [error.stack || error.message])
  } catch (e) {
    write('error', ['Could not get stack trace because:', e.stack || e.message])
  }

  nativeConsole.error(error)
}

function start() {
  // now that we've gotten the source map, we can run the tests
  var script = document.createElement("script")
  script.type = "text\/javascript"
  document.body.appendChild(script)
  script.src = 'script.js'
}