When I load https://meta.wikimedia.org/w/index.php?title=User:Fomafix/global.js&action=edit and fill in
console.log( "foobar" );
and click on Preview then I see foobar in the JavaScript console.
But when I fill in
console.log( "foo&bar" );
and click on Preview then I see the error message
SyntaxError: expected expression, got '}'
and all JavaScript of the UI is missing.
On the content
console.log('foobar');
the HTML of the preview contains
<script>(window.RLQ=window.RLQ||[]).push(function(){ mw.config.set({...}}); mw.loader.state({...}); mw.loader.implement("ext.globalCssJs.user@1xnrk9q",function($,jQuery,require,module){console.log('foobar');}); mw.loader.implement("user.options@03vsp6b",function($,jQuery,require,module){/*@nomin*/mw.user.options.set({...}); }); ... });</script>
On the content
console.log('foo&bar');
the HTML of the preview contains
<script>(window.RLQ=window.RLQ||[]).push(function(){ mw.config.set({...}}); mw.loader.state({...}); h(function(){mw.loader.implement("ext.globalCssJs.user@1q2pk0s",function($,jQuery,require,module){console.log('foo&bar');});});/*]] mw.loader.implement("user.options@03vsp6b",function($,jQuery,require,module){/*@nomin*/mw.user.options.set({...}); }); ... });</script>
Problem also happens when a < is in the string.
This may be a problem with the MediaWiki-ResourceLoader.