Define Constructor:

To define a custom class with constructor, add function name constructor as a class member:

Ext.define('Student', {
    name : 'unnamed',
    getName : function(){
        return 'Student name is ' + this.name;
    },
    constructor : function(studentName){
        if(studentName)
            this.name = studentName;
    }
});

Now you can create an object by passing parameter to the constructor:

var studentObj = Ext.create('Student','XYZ');
//or
var studentObj = new Student('XYZ');
studentObj.getName();
//output: Student name is XYZ

Try it on https://fiddle.sencha.com/#fiddle/3kv