extjs numberfield 不允许输入负数

allowExponential: false

Ext.application({
    name : 'Fiddle',

    launch : function() {
        Ext.create('Ext.form.Panel', {
            title: 'On The Wall',
            width: 300,
            bodyPadding: 10,
            renderTo: Ext.getBody(),
            items: [{
                xtype: 'numberfield',
                anchor: '100%',
                name: 'bottles',
                fieldLabel: 'Bottles of Beer',
                value: 99,
                maxValue: 99,
                minValue: 0,
                allowExponential: false,
                allowDecimals: true
            }],
            buttons: [{
                text: 'Take one down, pass it around',
                handler: function() {
                    this.up('form').down('[name=bottles]').spinDown();
                }
            }]
        });
    }
});