A tiny node.js debugging utility modelled after node core's debugging technique.
**Discussion around the V3 API is under way [here](https://github.com/visionmedia/debug/issues/370)**
## Installation
```bash
$ npm install debug
```
## Usage
`debug` exposes a function; simply pass this function the name of your module, and it will return a decorated version of `console.error` for you to pass debug statements to. This will allow you to toggle the debug output for different parts of your module as well as the module as a whole.
Example _app.js_:
```js
var debug = require('debug')('http')
, http = require('http')
, name = 'My App';
// fake app
debug('booting %s', name);
http.createServer(function(req, res){
debug(req.method + ' ' + req.url);
res.end('hello\n');
}).listen(3000, function(){
debug('listening');
});
// fake worker of some kind
require('./worker');
```
Example _worker.js_:
```js
var debug = require('debug')('worker');
setInterval(function(){
debug('doing some work');
}, 1000);
```
The __DEBUG__ environment variable is then used to enable these based on space or comma-delimited names. Here are some examples:
On Windows the environment variable is set using the `set` command.
```cmd
set DEBUG=*,-not_this
```
Note that PowerShell uses different syntax to set environment variables.
```cmd
$env:DEBUG = "*,-not_this"
```
Then, run the program to be debugged as usual.
## Millisecond diff
When actively developing an application it can be useful to see when the time spent between one `debug()` call and the next. Suppose for example you invoke `debug()` before requesting a resource, and after as well, the "+NNNms" will show you how much time was spent between calls.
If you're using this in one or more of your libraries, you _should_ use the name of your library so that developers may toggle debugging as desired without guessing names. If you have more than one debuggers you _should_ prefix them with your library name and use ":" to separate features. For example "bodyParser" from Connect would then be "connect:bodyParser".
## Wildcards
The `*` character may be used as a wildcard. Suppose for example your library has debuggers named "connect:bodyParser", "connect:compress", "connect:session", instead of listing all three with `DEBUG=connect:bodyParser,connect:compress,connect:session`, you may simply do `DEBUG=connect:*`, or to run everything using this module simply use `DEBUG=*`.
You can also exclude specific debuggers by prefixing them with a "-" character. For example, `DEBUG=*,-connect:*` would include all debuggers except those starting with "connect:".
## Environment Variables
When running through Node.js, you can set a few environment variables that will
Debug uses [printf-style](https://wikipedia.org/wiki/Printf_format_string) formatting. Below are the officially supported formatters:
| Formatter | Representation |
|-----------|----------------|
| `%O` | Pretty-print an Object on multiple lines. |
| `%o` | Pretty-print an Object all on a single line. |
| `%s` | String. |
| `%d` | Number (both integer and float). |
| `%j` | JSON. Replaced with the string '[Circular]' if the argument contains circular references. |
| `%%` | Single percent sign ('%'). This does not consume an argument. |
### Custom formatters
You can add custom formatters by extending the `debug.formatters` object. For example, if you wanted to add support for rendering a Buffer as hex with `%h`, you could do something like:
```js
const createDebug = require('debug')
createDebug.formatters.h = (v) => {
return v.toString('hex')
}
// …elsewhere
const debug = createDebug('foo')
debug('this is hex: %h', new Buffer('hello world'))
// foo this is hex: 68656c6c6f20776f726c6421 +0ms
```
## Browser support
You can build a browser-ready script using [browserify](https://github.com/substack/node-browserify),
or just use the [browserify-as-a-service](https://wzrd.in/) [build](https://wzrd.in/standalone/debug@latest),
if you don't want to build it yourself.
Debug's enable state is currently persisted by `localStorage`.
Consider the situation shown below where you have `worker:a` and `worker:b`,
and wish to debug both. You can enable this using `localStorage.debug`:
```js
localStorage.debug = 'worker:*'
```
And then refresh the page.
```js
a = debug('worker:a');
b = debug('worker:b');
setInterval(function(){
a('doing some work');
}, 1000);
setInterval(function(){
b('doing some work');
}, 1200);
```
#### Web Inspector Colors
Colors are also enabled on "Web Inspectors" that understand the `%c` formatting
option. These are WebKit web inspectors, Firefox ([since version
