Sample Code:

Component:

<aura:component implements="force:appHostable" >    
    <aura:attribute name = "defaultVal" type = "String"/>
        <aura:attribute name = "options" type = "List" default = "[
            {'label': 'Apple', 'value': 'apple'},
            {'label': 'Banana', 'value': 'banana'},
            {'label': 'Grapes', 'value': 'grapes'},
        ]"/>    
    <aura:handler name = "init" value = "{!this}" action = "{!c.onInit}"/>
    
    <div class = "slds-box slds-theme_default">
        <lightning:combobox
            name="Fruit"
            label = "Select a Fruit"
            value = "{! v.defaultVal }"
            placeholder = "Select Progress"
            options="{! v.options }"
            onchange="{! c.handleChange }"/>
    </div>
</aura:component>

JavaScript Controller:

({
    onInit : function( component, event, helper ) {    
          
        component.set( "v.defaultVal", "banana" );  
          
    },
    
    handleChange: function (cmp, event) {
        
        var selectedOptionValue = event.getParam( "value" );
        alert( "Value selected is " + selectedOptionValue );
        
    }
    
});

Output: