add instance extends feature (#524)

* instance extends feature

* add .extend documentation

* allow empty delimiter in extend

Author: kwolfy

README.md

@@ -268,6 +268,20 @@ error('now goes to stdout via console.info');
 log('still goes to stdout, but via console.info now');
 ```
 
+## Extend
+You can simply extend debugger 
+```js
+const log = require('debug')('auth');
+
+//creates new debug instance with extended namespace
+const logSign = log.extend('sign');
+const logLogin = log.extend('login');
+
+log('hello'); // auth hello
+logSign('hello'); //auth:sign hello
+logLogin('hello'); //auth:login hello
+```
+
 ## Set dynamically
 
 You can also enable debug dynamically by calling the `enable()` method :

src/common.js

@@ -123,6 +123,7 @@ module.exports = function setup(env) {
     debug.useColors = createDebug.useColors();
     debug.color = selectColor(namespace);
     debug.destroy = destroy;
+    debug.extend = extend;
     //debug.formatArgs = formatArgs;
     //debug.rawLog = rawLog;
 
@@ -146,6 +147,10 @@ module.exports = function setup(env) {
     }
   }
 
+  function extend (namespace, delimiter) {
+    return createDebug(this.namespace + (typeof delimiter !== 'undefined' ? delimiter : ':') + namespace);
+  }
+
   /**
    * Enables a debug mode by namespaces. This can include modes
    * separated by a colon and wildcards.

test/debug_spec.js

@@ -64,4 +64,30 @@ describe('debug', function () {
     });
   });
 
+
+  describe('extend namespace', function () {
+    var log;
+
+    beforeEach(function () {
+      debug.enable('foo');
+      log = debug('foo');
+    });
+
+    it('should extend namespace', function () {
+      var logBar = log.extend('bar');
+      expect(logBar.namespace).to.be.equal('foo:bar');
+    });
+
+    it('should extend namespace with custom delimiter', function () {
+      var logBar = log.extend('bar', '--');
+      expect(logBar.namespace).to.be.equal('foo--bar');
+    });
+
+    it('should extend namespace with empty delimiter', function () {
+      var logBar = log.extend('bar', '');
+      expect(logBar.namespace).to.be.equal('foobar');
+    });
+
+  });
+
 });