A tiny JavaScript debugging utility modelled after Node.js core's debugging
technique. Works in Node.js and web browsers.
## 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_](./examples/node/app.js):
```js
var debug = require('debug')('http')
, http = require('http')
, name = 'My App';
// fake app
debug('booting %o', 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_](./examples/node/worker.js):
```js
var a = require('debug')('worker:a')
, b = require('debug')('worker:b');
function work() {
a('doing lots of uninteresting work');
setTimeout(work, Math.random() * 1000);
}
work();
function workb() {
b('doing some work');
setTimeout(workb, Math.random() * 2000);
}
workb();
```
The `DEBUG` environment variable is then used to enable these based on space or
comma-delimited names.
Here are some examples:
<imgwidth="647"alt="screen shot 2017-08-08 at 12 53 04 pm"src="https://user-images.githubusercontent.com/71256/29091703-a6302cdc-7c38-11e7-8304-7c0b3bc600cd.png">
<imgwidth="647"alt="screen shot 2017-08-08 at 12 53 38 pm"src="https://user-images.githubusercontent.com/71256/29091700-a62a6888-7c38-11e7-800b-db911291ca2b.png">
<imgwidth="647"alt="screen shot 2017-08-08 at 12 53 25 pm"src="https://user-images.githubusercontent.com/71256/29091701-a62ea114-7c38-11e7-826a-2692bedca740.png">
#### Windows command prompt notes
##### CMD
On Windows the environment variable is set using the `set` command.
```cmd
set DEBUG=*,-not_this
```
Example:
```cmd
set DEBUG=* & node app.js
```
##### PowerShell (VS Code default)
PowerShell uses different syntax to set environment variables.
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". If you append a "*" to the end of your name, it will always be enabled regardless of the setting of the DEBUG environment variable. You can then use it for normal output as well as debug output.
## 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
| `%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);
```
In Chromium-based web browsers (e.g. Brave, Chrome, and Electron), the JavaScript console will—by default—only show messages logged by `debug` if the "Verbose" log level is _enabled_.
Will disable all namespaces. The functions returns the namespaces currently
enabled (and skipped). This can be useful if you want to disable debugging
temporarily without knowing what was enabled to begin with.
For example:
```js
let debug = require('debug');
debug.enable('foo:*,-foo:bar');
let namespaces = debug.disable();
debug.enable(namespaces);
```
Note: There is no guarantee that the string will be identical to the initial
enable string, but semantically they will be identical.
## Checking whether a debug target is enabled
After you've created a debug instance, you can determine whether or not it is
enabled by checking the `enabled` property:
```javascript
const debug = require('debug')('http');
if (debug.enabled) {
// do stuff...
}
```
You can also manually toggle this property to force the debug instance to be
enabled or disabled.
## Usage in child processes
Due to the way `debug` detects if the output is a TTY or not, colors are not shown in child processes when `stderr` is piped. A solution is to pass the `DEBUG_COLORS=1` environment variable to the child process.
For example:
```javascript
worker = fork(WORKER_WRAP_PATH, [workerPath], {
stdio: [
/* stdin: */ 0,
/* stdout: */ 'pipe',
/* stderr: */ 'pipe',
'ipc',
],
env: Object.assign({}, process.env, {
DEBUG_COLORS: 1 // without this settings, colors won't be shown
