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        <tr>
          <th>Bug ID</th>
          <td><a class="bz_bug_link 
          bz_status_NEW "
   title="NEW - &quot;use strict&quot;; delete ProgressEvent.length; should not throw"
   href="https://bugs.webkit.org/show_bug.cgi?id=158062">158062</a>
          </td>
        </tr>

        <tr>
          <th>Summary</th>
          <td>&quot;use strict&quot;; delete ProgressEvent.length; should not throw
          </td>
        </tr>

        <tr>
          <th>Classification</th>
          <td>Unclassified
          </td>
        </tr>

        <tr>
          <th>Product</th>
          <td>WebKit
          </td>
        </tr>

        <tr>
          <th>Version</th>
          <td>WebKit Nightly Build
          </td>
        </tr>

        <tr>
          <th>Hardware</th>
          <td>Unspecified
          </td>
        </tr>

        <tr>
          <th>OS</th>
          <td>Unspecified
          </td>
        </tr>

        <tr>
          <th>Status</th>
          <td>NEW
          </td>
        </tr>

        <tr>
          <th>Severity</th>
          <td>Normal
          </td>
        </tr>

        <tr>
          <th>Priority</th>
          <td>P2
          </td>
        </tr>

        <tr>
          <th>Component</th>
          <td>JavaScriptCore
          </td>
        </tr>

        <tr>
          <th>Assignee</th>
          <td>webkit-unassigned&#64;lists.webkit.org
          </td>
        </tr>

        <tr>
          <th>Reporter</th>
          <td>zcorpan&#64;gmail.com
          </td>
        </tr></table>
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        <div>
        <pre>See <a href="https://github.com/w3c/web-platform-tests/pull/3068">https://github.com/w3c/web-platform-tests/pull/3068</a>

This fails in WebKit:

&lt;!DOCTYPE html&gt;
&lt;script&gt;
&quot;use strict&quot;;
try {
  delete ProgressEvent.length;
  w('PASS');
} catch(e) {
  w('FAIL');
}
&lt;/script&gt;

<a href="http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4231">http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4231</a>

Per ES6 deleting a function's &quot;length&quot; property should apparently not throw (even in strict mode). It throws in WebKit for a vanilla function:

&lt;!DOCTYPE html&gt;
&lt;script&gt;
&quot;use strict&quot;;
var x = function(x) {};
try {
  delete x.length;
  w('PASS');
} catch(e) {
  w('FAIL');
}
&lt;/script&gt;

<a href="http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4232">http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4232</a>

Also, the ProgressEvent interface object is typeof &quot;object&quot; in WebKit but per WebIDL should be &quot;function&quot;.

&lt;!DOCTYPE html&gt;
&lt;script&gt;
w(typeof ProgressEvent == &quot;function&quot; ? &quot;PASS&quot; : &quot;FAIL&quot;);
&lt;/script&gt;

<a href="http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4233">http://software.hixie.ch/utilities/js/live-dom-viewer/saved/4233</a></pre>
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