instrument_js.rake.rb

namespace :test do
  task :instrument_js do
    `jscoverage public/javascripts/ public/instrumented/`
    `sed 's/javascripts/instrumented/g' app/views/layouts/layout.html.erb > TMPFILE && mv TMPFILE app/views/layouts/layout.html.erb`
  end
end

js_coverage_to_cobertura.rb

module JsCoverageToCobertura  
  def collect_coverage
    @@coverage ||= {}
    eval_function = <<EOF
if (! window.jscoverage_report) {
  window.jscoverage_report = function jscoverage_report(dir) {
    if(window._$jscoverage == undefined) return "";
    var pad = function (s) {
      return '0000'.substr(s.length) + s;
    };

    var quote = function (s) {
      return '"' + s.replace(/[\\u0000-\\u001f"\\\\\\u007f-\\uffff]/g, function (c) {
        switch (c) {
        case '\\b':
          return '\\\\b';
        case '\\f':
          return '\\\\f';
        case '\\n':
          return '\\\\n';
        case '\\r':
          return '\\\\r';
        case '\\t':
          return '\\\\t';
        case '"':
          return '\\\\"';
        case '\\\\':
          return '\\\\\\\\';
        default:
          return '\\\\u' + pad(c.charCodeAt(0).toString(16));
        }
      }) + '"';
    };

    var json = [];
    for (var file in window._$jscoverage) {
      var coverage = window._$jscoverage[file];

      var array = [];
      var length = coverage.length;
      for (var line = 0; line < length; line++) {
        var value = coverage[line];
        if (value === undefined || value === null) {
          value = 'null';
        }else{
          coverage[line] = 0; //stops double counting
        }
        array.push(value);
      }
      json.push(quote(file) + ':{"coverage":[' + array.join(',') + ']}');
    }
    json = '{' + json.join(',') + '}';

    return json;
  };
}; 
window.jscoverage_report()
EOF
    
    new_results = @selenium.js_eval(eval_function)
    
    unless new_results == ''
      new_results = new_results.parse_json
    
      new_results.each do |file, data|
        unless @@coverage[file]
          @@coverage[file] = data['coverage']
        else
          previous = @@coverage[file]
          data['coverage'].each_with_index do |a,index| 
            previous[index] += a.to_i if a
          end
        end
      end
    end
  end
  
  def generate_cobertura_xml
    return if @@coverage.none?
    total_lines = 0
    total_hits = 0
    class_text = ""
    @@coverage.each do |file, coverage_array|
      lines = []
      source_lines = 0
      hit_lines = 0
      coverage_array.each_with_index do |hits, line|
        if hits
          lines << "<line branch=\"false\" hits=\"#{hits}\" number=\"#{line}\"/>"
          source_lines += 1
          hit_lines += 1 if hits.to_i > 0
        end
      end
      total_lines += source_lines
      total_hits += hit_lines
      class_text += "<class branch-rate=\"0\" complexity=\"0.0\" filename=\"#{file}\" line-rate=\"#{hit_lines.to_f/source_lines rescue 0}\" name=\"#{file}\"><lines>\n        "
      class_text += lines.join("\n        ")
      class_text += "\n        </lines></class>"
    end
    javascript_dir = File.expand_path(File.join(File.dirname(__FILE__), '../../public/javascripts'))
    text = <<EOF 
<?xml version="1.0" ?>
<!DOCTYPE coverage
  SYSTEM 'http://cobertura.sourceforge.net/xml/coverage-03.dtd'>
<coverage>
  <sources>
    <source>
      #{javascript_dir}
    </source>
  </sources>
  <packages>
    <package branch-rate="0" complexity="0.0" line-rate="#{total_hits.to_f/total_lines rescue 0}" name="application">
      <classes>
      #{class_text}
      </classes>
    </package>
  </packages>
</coverage>
EOF
    
    File.open(File.join(File.dirname(__FILE__),"coverage.xml"), 'w') do |f|
      f.write text
    end
  end
end