this give me error msg if empty all checkboxes but run insert query and data inserted

enter code here

function checkbox(){

enter code here
function checkbox(){
            // body...

            var a=document.getElementsByName('proinf[]');
            for(var i=0; i < a.length; i++){
                if(a[i].checked==true){
                    document.getElementById("error_checkbox").innerHTML="";
                    return true;            
                }
                else{
                    document.getElementById("error_checkbox").innerHTML="* Please Checked Atleast 1 Professional Skill";


                }
            }
        }

<input id="sbmtbtn" type="Submit" class="btn btn-primary"    onclick="return checkbox()" name="Submit" value="Submit">

        var a=document.getElementsByName('proinf[]');
        for(var i=0; i < a.length; i++){
            if(a[i].checked==true){
                document.getElementById("error_checkbox").innerHTML="";
                return true;            
            }

            else

{

                document.getElementById("error_checkbox").innerHTML="* Please Checked Atleast 1 Professional Skill";        
            }
        }
    }