this Keyword

You can use 'this' keyword to refer to the current object.
this' refers to the calling object in a method

function validate(obj, lower, upper) { 
   if ( (obj.value < lower) || (obj.value > upper) ) { 
      alert('out of range'); 
   } 
} 

<input type="text" name="age" size="3" 
    onChange="validate(this, 18, 99)">