How to alter a CallSite?

[julien-f]

I am currently writing a modifier for stack-chain to enable source maps in Node and I need to alter some call sites to fix the filename, the line and the column.

Unfortunately, the CallSite#toString() is read-only, any ideas?

'use strict'

function getCallSite () {
  // Save original values.
  var stackTraceLimit = Error.stackTraceLimit
  var prepareStackTrace = Error.prepareStackTrace

  // Make the changes.
  Error.stackTraceLimit = 2
  Error.prepareStackTrace = function (_, callSites) {
    return callSites[1]
  }

  // Get the call site.
  var callSite = new Error().stack

  // Restore the original values.
  Error.stackTraceLimit = stackTraceLimit
  Error.prepareStackTrace = prepareStackTrace

  return callSite
}

var callSite = getCallSite()

callSite.getFilename = function () {
  return 'foo.js'
}

console.log(callSite.toString())

Expected result:

Object.<anonymous> (foo.js:22:16)

Actual result:

callSite.getFileName = function () {
                     ^

TypeError: Cannot assign to read only property 'getFileName' of [object Object]
    at Object.<anonymous> (/tmp/index.js:26:22)
    at Module._compile (module.js:434:26)
    at Object.Module._extensions..js (module.js:452:10)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Function.Module.runMain (module.js:475:10)
    at startup (node.js:117:18)
    at node.js:951:3

I also tried to clone the callSite object, but this time, the toString() method throws an illegal access error:

var copy = Object.create(callSite, {
  getFileName: {
    value: function () {
      return 'foo.js'
    }
  }
})

console.log(copy.toString())

[julien-f]

The work around currently used in source-map-support is to reimplement the toString() method, which looks pretty bad to me :/

[AndreasMadsen]

Hmm. I have never done that before. Looking quickly at it, it seams like defining your own CallSite object from scratch is the only way. (the CallSite object is defined here https://github.com/v8/v8/blob/263340113713db788e56abda61062dd9513b9f79/src/js/messages.js#L593L749)

I will have to investigate it more, but I don't think there is anything that can be done.

[julien-f]

Maybe stack-chain should provide an utility function to clone a CallSite with a custom toString().

It's far from perfect but at least it avoids the need to duplicate this code for each stack modifier :/

[AndreasMadsen]

I think a mutate helper in general would be nice. But I wouldn't restrict it to just toString()

[AndreasMadsen]

I would welcome a CustomCallSite constructor that looks like:

CustomCallSite(callSite, {
  fileName: 'script.coffee',
  // other overwritten properties
});

It would the have the v8-defined get and is functions, which uses the custom properties (in second argument) and defaults to using the callSite object. At last it would provide the v8-standard toString function that formats the CustomCallSite object.

Eventually I might do this myself, but I would suggests that you make a pull request.

[julien-f]

Ok, I don't have much time for it currently but maybe one day :)