How to use onkeyup event to validate in JavaScript

In this article i am going to explain How to use onkeyup event to validate in JavaScript.

Introduction

In your html page add the control you want to validate but for this sample I used textbox to demonstrate this code, in my textbox control i used onkeyup event to validate whether recently entered character is numeric or not. So you can call this JavaScript code as your requirement in your project.

Test.html

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<input type="text" id="txt" name="txt" onkeyup="txtValidate(this.id)"/> 

JavaScript Code

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    function txtValidate(txtid)
    {
    var ok=0;
    var a=document.getElementById(txtid).value;
       
       for(var i=0;i<=a.length-1;i++)
      {
        var j=a.charCodeAt(i);
           for(var k=48;k<=57;k++)
        {
          ok=0;
          if(k==j)
          {
            ok=1;
            break ;
          }
        }
       
      }
      if(ok==0)
      {
        alert("Only Numeric Values Allowed");
        document.getElementById(txtid).value="";
        for(var i=0;i<a.length-1;i++)
        {
         var j=a.charCodeAt(i);
           for(var k=48;k<=57;k++)
        {
          ok=0;
          if(k==j)
          {
           document.getElementById(txtid).value+=a.charAt(i);
           }
         } 
        }
      }
    }