pretty-error

See nodejs errors with less clutter

PrettyError

A small tool to see node.js errors with less clutter:

... which is more readable compared to node's unformatted errors:

Install with npm:

npm install pretty-error

To see an error rendered with colors, you can do this:

var PrettyError = require('pretty-error');
var pe = new PrettyError();
var renderedError = pe.render(new Error('Some error message'));
 
console.log(renderedError);

Of course, you can render caught exceptions too:

try {
   doSomethingThatThrowsAnError();
} catch (error) {
   console.log(pe.render(error));
}

But if you wanna see all node errors with colors, there is a shortcut for it:

require('pretty-error').start();

... which is essentially equal to:

var PrettyError = require('pretty-error');
 
// instantiate PrettyError, which can then be used to render error objects 
var pe = new PrettyError();
pe.start();

PrettyError turns error objects into something similar to an html document, and then uses the upcoming RenderKid to render the document using simple html/css-like commands. This allows PrettyError to be themed using simple css-like declarations.

PrettyError's default theme is a bunch of simple css-like declarations. Here is the source of the default theme.

Surely, you can change all aspects of this theme. Let's do a minimal one:

// the start() shortcuts returns an instance of PrettyError ... 
pe = require('pretty-error').start();
 
// ... which we can then use to customize with css declarations: 
pe.appendStyle({
   // this is a simple selector to the element that says 'Error' 
   'pretty-error > header > title > kind': {
      // which we can hide: 
      display: 'none'
   },
 
   // the 'colon' after 'Error': 
   'pretty-error > header > colon': {
      // we hide that too: 
      display: 'none'
   },
 
   // our error message 
   'pretty-error > header > message': {
      // let's change its color: 
      color: 'bright-white',
 
      // we can use black, red, green, yellow, blue, magenta, cyan, white, 
      // grey, bright-red, bright-green, bright-yellow, bright-blue, 
      // bright-magenta, bright-cyan, and bright-white 
 
      // we can also change the background color: 
      background: 'cyan',
 
      // it understands paddings too! 
      padding: '0 1' // top/bottom left/right 
   },
 
   // each trace item ... 
   'pretty-error > trace > item': {
      // ... can have a margin ... 
      marginLeft: 2,
 
      // ... and a bullet character! 
      bullet: '"<grey>o</grey>"'
 
      // Notes on bullets: 
      // 
      // The string inside the quotation mark will be used for bullets. 
      // 
      // You can set its color/background color using tags. 
      // 
      // This example sets the background color to white, and the text color 
      // to cyan, the character will be a hyphen with a space character 
      // on each side: 
      // example: '"<bg-white><cyan> - </cyan></bg-white>"' 
      // 
      // Note that we should use a margin of 3, since the bullet will be 
      // 3 characters long. 
   },
 
   'pretty-error > trace > item > header > pointer > file': {
      color: 'bright-cyan'
   },
 
   'pretty-error > trace > item > header > pointer > colon': {
      color: 'cyan'
   },
 
   'pretty-error > trace > item > header > pointer > line': {
      color: 'bright-cyan'
   },
 
   'pretty-error > trace > item > header > what': {
      color: 'bright-white'
   },
 
   'pretty-error > trace > item > footer > addr': {
      display: 'none'
   }
});

Here is how our minimal theme will look like:

I'll post more examples on RenderKid when it comes out of beta.

There are a few methods to help you customize the contents of your error logs.

Let's instantiate first:

PrettyError = require('pretty-error');
pe = new PrettyError();
 
// or: 
pe = require('pretty-error').start();

You might want to substitute long paths with shorter, more readable aliases:

pe.alias('E:/open-source/theatrejs/scripts/js', '(Theare.js)');
 
// to remove the alias: 
pe.removeAlias('E:/open-source/theatrejs/scripts/js');
 
// or: 
pe.removeAllAliases();

You might want to skip trace lines that belong to specific packages (chai, when, socket.io):

pe.skipPackage('chai', 'when', 'socket.io');
 
// to unskip: 
pe.unskipPackage('socket.io');
pe.unskipAllPackages();
// this will skip node.js, path.js, event.js, etc. 
pe.skipNodeFiles();
 
// also: 
pe.unskipNodeFiles();
pe.skipPath('/home/dir/someFile.js');
 
// also: 
pe.unskipPath('/home/dir/someFile.js');
pe.unskipAllPaths();

You can customize which trace lines get logged and which won't:

pe.skip(function(traceLinelineNumber){
   // if we know which package this trace line comes from, and it isn't 
   // our 'demo' package ... 
   if (typeof traceLine.packageName !== 'undefined' && traceLine.packageName !== 'demo') {
      // then skip this line 
      return true;
   }
 
   // You can console.log(traceLine) to see all of it's properties. 
   // Don't expect all these properties to be present, and don't assume 
   // that our traceLine is always an object. 
});
 
// there is also: 
pe.unskip(fn);
pe.unskipAll();
pe.filter(function(traceLinelineNumber){
   // the 'what' clause is something like: 
   // 'DynamicTimeline.module.exports.DynamicTimeline._verifyProp' 
   if (typeof traceLine.what !== 'undefined'){
 
      // we can shorten it with a regex: 
      traceLine.what = traceLine.what.replace(
         /(.*\.module\.exports\.)(.*)/, '$2'
      );
   }
});
 
// there is also: 
pe.removeFilter(fn);
pe.removeAllFilters();
pe.withoutColors(); // Errors will be rendered without coloring 

PrettyError is very simple to set up, so it should be easy to use within other frameworks.

Integrating with express

Most frameworks such as express, catch errors automatically and provide a mechanism to handle those errors. Here is an example of how you can use PrettyError to log unhandled errors in express:

// this is app.js 
 
var express = require('express');
var PrettyError = require('pretty-error');
 
var app = express();
 
app.get('/', function(reqres) {
   // this will throw an error: 
   var a = b;
});
 
var server = app.listen(3000, function(){
   console.log('Server started \n');
});
 
 
// we can now instantiaite Prettyerror: 
pe = new PrettyError();
 
// and use it for our app's error handler: 
app.use(function(errreqresnext){
   console.log(pe.render(err));
});
 
// we can optionally configure prettyError to simplify the stack trace: 
 
pe.skipNodeFiles(); // this will skip events.js and http.js and similar core node files 
pe.skipPackage('express'); // this will skip all the trace lines about express` core and sub-modules 

This project has been out there for a while and used by fellow devs, but I still consider it a work in progress. Please let me know if something isn't working, or if you have any suggestions. And pull requests are of course, very welcome!

  • If you're on windows, you can get better typography by using an alternative console. I use ConEmu.
  • Also check out PrettyMonitor if you're using when.js. It's PrettyError wrapped to report unhandled when.js rejections.

MIT