RReverser's

Ingvar Stepanyan

JavaScript developer, speaker and reverse engineer. D2D programmer. Sometimes human.


displayName in error stack traces of V8

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Show custom function names in error stack traces

Firstly there was idea...

Chromium, Firefox and Safari agreed and implemented ability to set custom string as function name via non-standard property displayName on function instance.

You can see those custom names in debugger stack and so easier track source of error in long traces of anonymous functions.

Compare informativeness of error stack traces without and with displayName on example of jBinary:

Before vs After

And it's usage was simple

var f = function () {  
  throw new Error();
};

f.displayName = "super puper function";

f(); // enjoy descriptive stack trace!  

But not in Node.js

Since this property was implemented not in JS core but on level of developer tools, in Node.js you still get something like:
before which isn't too descriptive about what, where and why happened.

This drop-in library stringifies error stack traces in V8 both in browser and Node.js, simulating standard formatting but respecting displayName property, so when error occurs, you get stylish stack trace with custom function names (in case of jBinary those are type descriptors and field names being processed):

after

More crazy ideas to use this for?

You can even implement own micro BDD testing framework (example taken from http://visionmedia.github.io/mocha/#bdd-interface):

var assert = require('assert');

var describe = it = function (name, callback) {  
  callback.displayName = name;
  callback();
};

describe('Array', function () {  
  describe('#indexOf()', function () {
    it('should return -1 when not present', function () {
      assert.equal([1,2,3].indexOf(4), -2); // wrong!
    });
  });
});

bdd error

Installation - boring as usual

In Node.js

Simply install with npm as:

npm install stack-displayname  

and require it:

require('stack-displayname');  

In Browser

This script might be useful only in Chromium, where DevTools already respect displayName, but in the case you want to have custom function names in err.stack property of any Error instances (that's not natively supported), it's possible - just include script on the page via regular tag:

<script src="stack-displayname/displayName.js"></script>  

Non-supported browsers will just ignore it and show stack traces as usual.

License

Copyright 2014 Ingvar Stepanyan

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