<form name="myForm"> <input type="text" name="phoneNumber" onchange="checkPhone (this.value)" /> </form>
this is used to suggest “this input element,” and value is a property that stores the value associated with this form element. You could also access that same value as the following in order to pass along the value.
<form name="myForm"> <input type="text" name="phoneNumber" onchange="checkPhone (document.myForm.phoneNumber The this Keyword.value)" /> </form>
Note that the name of the form and the name of the element are both vital in getting at the correct value. That’s one reason for the this keyword; another is simple convenience