_javascript Techniques for writing JavaScript three different location codes

The following list of three different places to write JavaScript code, the effect is to click Button buttons pop-up alert warning box

The first is the most common, with the following code
HTML code
<input type= "button" value= "1" id= "Btn1" onclick= "Pop ()" >
JS Code
function Pop ()
{
Alert ("Call at JavaScript function");
}

The second is the simplest way to implement the code as follows
<input type= "button" value= buttons 2 "id=" btn2 "onclick=" Javascript:alert (' direct write function '); " >

The third approach is relatively complex, with the following code
HTML code
<input type= "button" value= "3" id= "Btn3" >
JS Code
var Obj=document.getelementbyid ("Btn3");//The following statement must be placed under the definition btn3, otherwise the compiler will not recognize BTN3.
if (Window.addeventlistener)/Mozilla, Netscape, Firefox and other browsers
{
Obj.addeventlistener ("click", Fun,false);/Note the false here
}
else//ie Browser
{
Obj.attachevent ("onclick", fun);
}
function Fun ()
{
Alert ("By triggering an event in a function");
}

Summary: Three ways to achieve the effect of writing is exactly the same, it should be said that three kinds of methods are commonly used, and each has advantages and disadvantages ....