aura:handler Init in Salesforce Lightning

This is also called as the constructor of lightning component. The init method is called once the component construction is over, as an example if I am creating a simple registration page and I want to display some default text value after page load in my input boxes I can do this by calling the javascript controller method and setting some default value in the attributes.
                          
Basic Syntax:

<aura:handler  name="init" value="{!this}" action="{!c.doinit}"/>

Let's take an example where we will be setting some default values in map type attribute and then displaying it.

Step 1) Build a lightning Component,

<aura:component controller="Mapvalue"  >

    <aura:handler  name="init" value="{!this}" action="{!c.doinit}"/>

        <aura:attribute type="map" name="Mapuse"/>  // Map type attribute to hold map value

       

        {!v.Mapuse.key1} 

        {!v.Mapuse.key2}

        {!v.Mapuse.key3}

</aura:component>

Note: The init method get's call after the construction of component is over and will call doinit method in javascript controller.

Step 2: Under javascript controller.

({

 doinit : function(component, event, helper) {    //  component, event, helper are parameter                                                                                                of doinit function

       

      var action=  component.get('c.getmymap');  // Calling apex method

        action.setCallback(this,function(response)   // getting response back from apex method

        {

                  var state=response.getState();            // getting the state

        if(state==='SUCCESS')

        {

            component.set('v.Mapuse',response.getReturnValue());    // setting the value in attribute

           

        }

                         

                         

        });

  $A.enqueueAction(action);

 }

})

Step 3:Build apex controller.

public class Mapvalue {

    @AuraEnabled     // Method must be AuraEnabled in apex

    public static map<string,string> getmymap()

    {

        map<string,string> putkeyvalue= new map<string,string>();

        putkeyvalue.put('key1', 'Value1');

         putkeyvalue.put('key2', 'Value2');

         putkeyvalue.put('key3', 'Value3');

        return putkeyvalue;

    }

}

Step 4: Previewing with application.

<aura:application extends="force:slds">

    <c:Firstlightningcomponent/>

</aura:application>

After preview,