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Error 'undefined' is null or not an object in Internet Explorer

Question:

I am developing a web application using ExtJS and when I open my URL in Firefox (2), everything looks good and works well. In Internet Explorer (6) I get an error message

Error 'undefined' is null or not an object

Internet Explorer Javascript Error Message



Any ideas?

Answer:

I had the same error once. Look for any array of properties that has a trailing , (comma).
This can happen easily in ExtJS source code where frequently you will have a list of comma separated properties, then you delete the last one of those properties and you forget to remove the trailing comma.

Firefox doesn't complain, but Internet Explorer throws a cryptic error message.

The worst issue is that Internet Explorer gives you a line number and character position that seems worth nothing. It doesn't even tell you, in which source file your bug is.

Do you see the buggy comma in the example below? I marked it red, yet it's still not easy to spot. I guess my source code is messy :-|

  gridFruits = new Ext.grid.EditorGridPanel({
                         store: storeFruits,
                         cm: new Ext.grid.ColumnModel([
                          {header:"Fruit", width: 30, sortable:true, dataIndex:'group',
                                 editor:new Ext.form.TextField({allowBlank:false })},
 
                          {header:"Vitamin", width: 15, sortable:true, dataIndex:'vitamin'},
 
                          {header:"Profile", width: 15, sortable:true, dataIndex:'profile',
                                 renderer: displayProfile},
                         ]),
                         viewConfig: { forceFit: true },
                         sm: new Ext.grid.RowSelectionModel({singleSelect:true}),
                         clicksToEdit:2,
                         height: 444
                     });
 

Generated 20:02:09 on May 26, 2017