title: Javascript: Cannot assign to read only property '_epoch' of false
body: <p>I have a method to process the response from my google javascript client (gapi):</p>
  
  <pre><code>var processResponse = function(response) {
                result._state = 'loaded';
                response._epoch = (new Date()).getTime();
                ...
  </code></pre>
  
  <p>A few times I have gotten the following error:</p>
  
  <pre><code>TypeError: Cannot assign to read only property '_epoch' of false
      at processResponse (http://0.0.0.0:9000/scripts/services/haparaapi.js:110:31)
      at wrappedCallback (http://0.0.0.0:9000/bower_components/angular-scenario/angular-scenario.js:20892:81)
      at http://0.0.0.0:9000/bower_components/angular-scenario/angular-scenario.js:20978:26
      at Scope.$eval (http://0.0.0.0:9000/bower_components/angular-scenario/angular-scenario.js:21967:28)
      at Scope.$digest (http://0.0.0.0:9000/bower_components/angular-scenario/angular-scenario.js:21796:31)
      at Scope.$apply (http://0.0.0.0:9000/bower_components/angular-scenario/angular-scenario.js:22071:24)
      at http://0.0.0.0:9000/bower_components/angular-gapi/modules/gapi-client.js:121:32
      at https://apis.google.com/_/scs/apps-static/_/js/k=oz.gapi.en_GB.hE_reuZ6VdE.…/ed=1/am=AQ/rs=AGLTcCPj66Crj6soG8dKJE8lBSc_RPXXKA/cb=gapi.loaded_0:604:138
      at https://apis.google.com/_/scs/apps-static/_/js/k=oz.gapi.en_GB.hE_reuZ6VdE.…/ed=1/am=AQ/rs=AGLTcCPj66Crj6soG8dKJE8lBSc_RPXXKA/cb=gapi.loaded_0:579:311
      at Object.&lt;anonymous&gt; (https://apis.google.com/_/scs/apps-static/_/js/k=oz.gapi.en_GB.hE_reuZ6VdE.…1/ed=1/am=AQ/rs=AGLTcCPj66Crj6soG8dKJE8lBSc_RPXXKA/cb=gapi.loaded_0:163:76) 
  </code></pre>
  
  <p>This bug doesn't usually happen so I haven't managed to log what the response actually looked like. </p>
  
  <p>What does the error mean? Should I not be assigning values to the response?</p>