How can we define a constructor in a class in ExtJS?

The constructors are special method that are executed when a class is instantiated. You can use constructor to prepare the object in any way required. For example, to set up the default property values.

eg:

Ext.define('Test.Emp', {

   config: {

        name: 'Raja',

        gender: 'Male'

   },

   constructor: function(config){

       // initialize our config object

       this.initConfig(config);

    },

    getDetails: function(){

      alert('My name is ' + this.name);

    }

});

var honey = Ext.create("Test.Emp",{

name: 'Mary',

       gender: 'Female'

});