implemented filters

Author: Chris927

README.md

@@ -37,6 +37,7 @@ There are two different ways to use winston: directly via the default logger, or
   * [Working with multiple Loggers in winston](#working-with-multiple-loggers-in-winston)
   * [Using winston in a CLI tool](#using-winston-in-a-cli-tool)
   * [Extending another object with Logging](#extending-another-object-with-logging)
+  * [Filters](#filters)
 * [Working with transports](#working-with-transports)
 	* [Adding Custom Transports](#adding-custom-transports)
 * [Installation](#installation)
@@ -627,6 +628,26 @@ Often in a given code base with lots of Loggers it is useful to add logging meth
   myObject.info("127.0.0.1 - there's no place like home");
 ```
 
+### Filters
+Filters allow modifying the contents of log messages, e.g. to mask data that
+should not appear in logs.
+
+``` js
+logger.addFilter(function(msg) {
+  return maskCardNumbers(msg);
+});
+logger.info('transaction with card number 123456789012345 successful.');
+```
+
+This may result in this output:
+
+```
+info: transaction with card number 123456****2345 successful.
+```
+
+See [log-filter-test.js](./test/log-filter-test.js), where card number masking
+is implemented as an example.
+
 ## Working with Transports
 There are many transports supported by winston core. If you have a transport you would like to add either open an issue or fork and submit a pull request. Commits are welcome, but I'll give you extra street cred if you __add tests too :D__
 

lib/winston.js

@@ -78,7 +78,8 @@ var methods = [
   'cli',
   'handleExceptions',
   'unhandleExceptions',
-  'addRewriter'
+  'addRewriter',
+  'addFilter'
 ];
 common.setLevels(winston, null, defaultLogger.levels);
 methods.forEach(function (method) {

lib/winston/logger.js

@@ -51,6 +51,7 @@ var Logger = exports.Logger = function (options) {
   //
   this.transports        = {};
   this.rewriters         = [];
+  this.filters           = [];
   this.exceptionHandlers = {};
   this.profilers         = {};
   this._names            = [];
@@ -153,6 +154,10 @@ Logger.prototype.log = function (level) {
     meta = rewriter(level, msg, meta);
   });
 
+  this.filters.forEach(function(filter) {
+    msg = filter(msg);
+  });
+
   //
   // For consideration of terminal 'color" programs like colors.js,
   // which can add ANSI escape color codes to strings, we destyle the
@@ -472,6 +477,15 @@ Logger.prototype.addRewriter = function (rewriter) {
   this.rewriters.push(rewriter);
 }
 
+//
+// ### function addFilter (filter)
+// #### @filter {function} Filter function, called with the message as single
+// argument, expected to return the filtered message.
+//
+Logger.prototype.addFilter = function (filter) {
+  this.filters.push(filter);
+}
+
 //
 // ### function clear ()
 // Remove all transports from this instance

test/log-filter-test.js

@@ -0,0 +1,51 @@
+/* log-filter-test.js: Test filtering of message content in winston.
+ * (c) 2015 Chris Oloff
+ * MIT LICENSE
+ */
+
+var assert = require('assert'),
+    vows = require('vows'),
+    winston = require('../lib/winston'),
+    helpers = require('./helpers');
+
+/* To demo a filter, we filter out credit card numbers, assuming that a credit
+ * card number consists of 13 or more digits (without spaces). This function
+ * does the actual masking.
+ */
+function maskCardNumbers(s) {
+  var match;
+  while (match = s.match(/(\d{13}\d*)/)) {
+    var toBeMasked = match[1];
+    s = s.replace(toBeMasked, toBeMasked.substring(0,6) + '****' + toBeMasked.substring(toBeMasked.length - 4));
+  }
+  return s;
+}
+
+vows.describe('winston/logger/filter').addBatch({
+  "An instance of winston.Logger": {
+    topic: new (winston.Logger)({transports: [
+      new (winston.transports.Console)({ level: 'info' })
+    ]}),
+    "the addFilter() method": {
+      topic: function (logger) {
+        logger.addFilter(function (msg) {
+          return maskCardNumbers(msg);
+        });
+        return logger;
+      },
+      "should add the filter": function(logger) {
+        assert.equal(helpers.size(logger.filters), 1);
+      },
+      "the log() method": {
+        topic: function (logger) {
+          logger.once('logging', this.callback);
+          logger.log('info', 'card number 123456789012345 for testing');
+        },
+        "should filter the card number": function (transport, level, msg, meta) {
+          assert.equal(msg, 'card number 123456****2345 for testing');
+        }
+      }
+    }
+  }
+}).export(module);
+