You can't do it that way because you're expecting to to merge the objects which it just won't do.
Instead, you'll want to look at something like this (untested):
Ext.define('Base', {
extend: 'Ext.data.Store',
autoLoad: false,
constructor: function(config) {
// applyIf means only copy if it doesn't exist
Ext.applyIf(config, {
proxy: this.createProxy()
});
this.callParent([config]);
},
createProxy: function() {
return {
reader: {
type: 'json',
root: 'data',
totalProperty: 'total',
successProperty: 'success',
messageProperty: 'message'
},
writer: {
type: 'json',
encode: true,
writeAllFields: true,
root: 'data',
allowSingle: false
},
simpleSortMode: true
}
}
});
Ext.define('Sub', {
extend: 'Base',
createProxy: function(){
var proxy = this.callParent();
proxy.api = {
create: 'create',
update: 'update'
};
return proxy;
}
});