This is a typical beginner's problem.
You can do either of two thing.
1. When one validation fails, alert the message and return immediately. i.e.
if(isNaN(value)==true)
{
alert("Numeric value is expected");
return false;
}
2. As you wanted, gather all error message in one variable and alert it. i.e.
var msg="";
if(isNaN(value1)==true)
{
msg+="Input Field1 must have to be numeric";
}
if(value2=='')
{
msg+="\nInput Field2 cannot be empty";
}
if(msg=="")
return true;
alert(msg);
return false;
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