ExtJS 4: Uncaught TypeError: Cannot call method ‘substring’ of undefined ext-all-debug line 4384

When working with ExtJS 4, you’ll often run into ambiguous error message like this one:

Uncaught TypeError: Cannot call method ‘substring’ of undefined

ext-all-debug.js line 4384

I recently ran into this while coding up an Ext application for another blog post, and I thought I’d share with you its root cause and solution.

If you head straight to the line number in question, you’ll find yourself somewhere in the middle of the parseNamespace method of Ext’s ClassManager object and you’ll notice that the parameter that the method expects (namespace) is undefined, causing the method to throw an error. However if you look a little deeper in the callstack, you’ll notice that the instantiateByAlias method was called just before parseNamespace:.

instantiateByAlias: function() {
    var alias = arguments[0],
        args = arraySlice.call(arguments),
        className = this.getNameByAlias(alias);

    if (!className) {
        className = this.maps.aliasToName[alias];


    args[0] = className;

    return this.instantiate.apply(this, args);

If you place a breakpoint in this method and run the application again, you’ll be able to watch it load each Ext class (as well as your application’s custom components) by its xtype alias. Eventually, it attempt to load one of your application’s classes and then crash.

The reason for this crash is because you’ve most likely misspelled the alias of that component while referencing it in another component. Take a look at my grid code:

Ext.define('App.view.person.Grid', {
    extend: 'Ext.grid.Panel',
    alias: 'widget.persongrid',
    title: 'People',
    columns: [{
        header: 'ID',
        dataIndex: 'id'
    }, {
        header: 'First Name',
        dataIndex: 'first_name'
    }, {
        header: 'Last Name',
        dataIndex: 'last_name'
    }, {
        header: 'Address',
        dataIndex: 'address'
    }, {
        header: 'City',
        dataIndex: 'city'
    }, {
        header: 'State',
        dataIndex: 'state'
    }, {
        header: 'Zip Code',
        dataIndex: 'zip_code'

Now take a look at my viewport code:

Ext.define('App.view.Viewport', {
    extend: 'Ext.container.Viewport',
    config: {
        items: [{
            xtype: 'peoplegrid',
            store: 'People'

See how I referenced my Person grid view by the alias peoplegrid in the viewport file but I declared the alias to be persongrid in the grid file? That’s what’s causing the JavaScript error. So, the moral of the story is: If you get obscure errors about strings being undefined in ExtJS 4, always check deeper in the callstack (not just the line-number in question) and always double-check your xtype aliases across your application.

Hope this helps.