How to validate a field value in lighting component

       Validating form is more over important in UI development. Here is the simple example for validating a number field in lightning component app. 

Please take a look on the below link, if you don't know about creating lighting component.

Build Your First Lightning Component App In Salesforce

Lightning Component Code:

1. <aura:component >
2.     <center>
3.         <br/>
4.         <h5> <b> Age Validator </b> </h5> <br/>
5.         Enter your age: <ui:inputNumber aura:id="inputCmp"/> <br/>
6.         <ui:button label="Submit" press="{!c.validateAge}"/>
7.     </center>
8. </aura:component>

Component Controller Code:

1. {
2.     validateAge : function(component) {
3.         var ageField = component.find("inputCmp");
4.         var ageValue = ageField.get("v.value");
5.        
6.         if(isNaN(ageValue) || ageValue == '')
7.         {
8.             ageField.set("v.errors", [{message:"Enter a valid age."}]);
9.         }
10.         else if(parseInt(ageValue) > 150)
11.         {
12.             ageField.set("v.errors", [{message:"This is not a valid age."}]);
13.         }
14.         else if(parseInt(ageValue) < 18)
15.         {
16.             ageField.set("v.errors", [{message:"Minimum age to submit this form should be greater or equal 18"}]);
17.         }
18.         else
19.         {
20.             ageField.set("v.errors", null);
21.         }
22.     }
23. }

Output: