diff --git a/.gitignore b/.gitignore
index af197af..07966d4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,4 +5,5 @@
 /test/playground.html
 eruda*
 /script/icomoon/
+/src/**/*.js.map
 npm-debug.log
\ No newline at end of file
diff --git a/doc/CONTRIBUTING.md b/doc/CONTRIBUTING.md
index 7603bd3..98b3643 100644
--- a/doc/CONTRIBUTING.md
+++ b/doc/CONTRIBUTING.md
@@ -22,6 +22,8 @@ npm run dev
 npm run build
 # lint, build and test.
 npm run ci
+# check test coverage.
+npm run cov
 ```
 
 ## Project Structure
diff --git a/karma.conf.js b/karma.conf.js
index 391d3da..2f47399 100644
--- a/karma.conf.js
+++ b/karma.conf.js
@@ -1,4 +1,5 @@
-var webpackCfg = require('./script/webpack.release');
+var webpackCfg = require('./script/webpack.dev');
+webpackCfg.devtool = 'inline-source-map';
 
 module.exports = function (config)
 {
@@ -28,16 +29,26 @@ module.exports = function (config)
             'karma-phantomjs-launcher',
             'karma-wrap-preprocessor',
             'karma-coverage',
-            'karma-webpack'
+            'karma-webpack',
+            'karma-sourcemap-loader',
+            'karma-sourcemap-writer'
         ],
+        webpackServer: {
+            noInfo: true
+        },
         preprocessors: {
             'test/*.js': ['wrap'],
-            'src/index.js': ['webpack', 'coverage']
+            'src/index.js': ['webpack', 'sourcemap', 'sourcemap-writer', 'coverage']
         },
         webpack: webpackCfg,
         wrapPreprocessor: {
             template: '(function () { <%= contents %> })()'
         },
+        coverageReporter: {
+            type: 'json',
+            subdir: '.',
+            file: 'coverage-final.json'
+        },
         reporters: ['progress', 'coverage'],
         port: 9876,
         colors: true,
diff --git a/package.json b/package.json
index 3716a7a..c3cdc6f 100644
--- a/package.json
+++ b/package.json
@@ -6,6 +6,7 @@
   "scripts": {
     "ci": "npm run lint && npm run test && npm run build",
     "build": "webpack --config script/webpack.dev.js && webpack -p --config script/webpack.release.js",
+    "cov": "npm run test && remap-istanbul -i coverage/coverage-final.json -o coverage/coverage-remapped.json -t json && node script/coverage",
     "dev": "webpack-dev-server --config script/webpack.dev.js --host 0.0.0.0",
     "test": "karma start",
     "setup": "node script/cpTestLib",
@@ -59,6 +60,7 @@
     "eslint": "^4.5.0",
     "handlebars": "^4.0.5",
     "handlebars-loader": "^1.1.4",
+    "istanbul": "^0.4.5",
     "jasmine-core": "^2.4.1",
     "jasmine-jquery": "^2.1.1",
     "jquery": "^3.1.0",
@@ -69,12 +71,15 @@
     "karma-jasmine": "^1.1.0",
     "karma-jquery": "^0.2.2",
     "karma-phantomjs-launcher": "^1.0.4",
+    "karma-sourcemap-loader": "^0.3.7",
+    "karma-sourcemap-writer": "^0.1.2",
     "karma-webpack": "^2.0.5",
     "karma-wrap-preprocessor": "^0.1.0",
     "modernizr": "^3.3.1",
     "node-sass": "^4.5.0",
     "postcss-class-prefix": "^0.3.0",
     "postcss-loader": "^2.0.6",
+    "remap-istanbul": "^0.9.5",
     "sass-loader": "^6.0.3",
     "webpack": "^3.5.4",
     "webpack-dev-server": "^2.7.1"
diff --git a/script/coverage.js b/script/coverage.js
new file mode 100644
index 0000000..cdadd4f
--- /dev/null
+++ b/script/coverage.js
@@ -0,0 +1,30 @@
+var path = require('path'),
+    util = require('./util'),
+    istanbul = require('istanbul');
+
+var collector = new istanbul.Collector(),
+    reporter = new istanbul.Reporter();
+
+var remappedJson = require('../coverage/coverage-remapped.json');
+
+var coverage = util.reduce(util.keys(remappedJson), function (result, source) 
+{
+    if (source.match(/src.*\.js$/) && source.indexOf('node_modules') < 0) 
+    {
+        var correctPath = source.replace(path.resolve(__dirname, '../src'), path.resolve(__dirname, '../'));
+
+        var val = remappedJson[source];
+        val.path = correctPath;
+        result[correctPath] = val;
+    }
+
+    return result;
+}, {});
+
+collector.add(coverage);
+
+reporter.add('html');
+reporter.write(collector, true, function ()
+{
+    console.log('open coverage/index.html to see the coverage report.');
+});
\ No newline at end of file
diff --git a/script/util.js b/script/util.js
index 9af1fc7..a6f7a11 100644
--- a/script/util.js
+++ b/script/util.js
@@ -145,6 +145,12 @@
          * |------|-------|-------------------------------------|
          * |value |*      |Value to check                       |
          * |return|boolean|True if value is correctly classified|
+         * 
+         * ```javascript
+         * isNum(5); // -> true
+         * isNum(5.1); // -> true
+         * isNum({}); // -> false
+         * ```
          */
 
         /* dependencies
@@ -499,5 +505,66 @@
         return exports;
     })();
 
+    /* ------------------------------ reduce ------------------------------ */
+
+    _.reduce = (function ()
+    {
+        /* Turn a list of values into a single value.
+         *
+         * |Name               |Type        |Desc                          |
+         * |-------------------|------------|------------------------------|
+         * |obj                |object array|Collection to iterate over    |
+         * |[iteratee=identity]|function    |Function invoked per iteration|
+         * |[initial]          |*           |Initial value                 |
+         * |[ctx]              |*           |Function context              |
+         * |return             |*           |Accumulated value             |
+         *
+         * ```javascript
+         * reduce([1, 2, 3], function (sum, n) { return sum + n }, 0); // -> 6
+         * ```
+         */
+
+        /* dependencies
+         * optimizeCb isArrLike isUndef keys 
+         */
+
+        function exports(obj, iteratee, initial, ctx)
+        {
+            iteratee = optimizeCb(iteratee, ctx);
+
+            var i = 0, len, key;
+
+            if (isArrLike(obj))
+            {
+               if (isUndef(initial))
+               {
+                   initial = obj[0];
+                   i = 1;
+               }
+               for (len = obj.length; i < len; i++)
+               {
+                   initial = iteratee(initial, obj[i], i, obj);
+               }
+            } else
+            {
+                var _keys = keys(obj);
+                if (isUndef(initial))
+                {
+                    initial = obj[_keys[0]];
+                    i = 1;
+                }
+                for (len = _keys.length; i < len; i++)
+                {
+                    key = _keys[i];
+                    initial = iteratee(initial, obj[key], key, obj);
+                }
+            }
+
+            return initial;
+        }
+
+        return exports;
+    })();
+
     return _;
 }));
\ No newline at end of file
diff --git a/src/index.js b/src/index.js
index ce370a0..525af1e 100644
--- a/src/index.js
+++ b/src/index.js
@@ -191,3 +191,5 @@ module.exports = {
 };
 
 extraUtil(util);
+
+//# sourceMappingURL=index.js.map
\ No newline at end of file