A few examples of this mechanism inside JavaScript

The This value in JavaScript changes depending on the use of the scene, but there is always a principle that this will always point to the object that is currently calling the function. I'll give you a few examples to illustrate the problem.
The 1.this itself always points to an instance of the current class
function ShowMsg () {
var msg=1;
alert (this.msg);
}
ShowMsg ();
This is called when the undefined is displayed, because ShowMsg has no instance, so it cannot be used
2. Let's look at a click event
<div id= "nav" onclick= "getId ();" >ddd</div>
function GetId () {
Alert (this.id)
}

The result of the run in the segment code shows that this.id is undefined, judging who the this is pointing to, and when it is not defined at execution time. As long as the function is not bound to use on the object, then this is the window. Inside this is the inside of the GetID function, but not the div inside the DOM.
This code is equivalent to Div.onclick = function () {getId ();}
There are several workarounds
A.
<div id= "NAV" >ddd</div>
Window.onload=initform;
function initform ()
{
document.getElementById ("Nav"). Onclick=getid;
}
function GetId () {
Console.log (This)
Alert (this.id)
}

In this code, the function is bound to the object. So the result is feasible.
B.
<div id= "nav" onclick= "getId (this);" >ddd</div>
function GetId (elem)
{
alert (elem.id);
}

This code is equivalent to passing this to Elem, Elem.id is available.
3. For the second example of the C method, I would like to say something more
<div id= "nav" onclick= "getId (this);" Xid= "Weixin" >ddd</div>
function GetId (elem)
{
Console.log (Elem)
alert (elem.id); This is a possible ID is an attribute in HTML
alert (ELEM.XID); But this is not possible because XID is not an intrinsic property of $ (item). attr ("XID")
Alert (this.id)//This is not possible
}

4. You can look at this example again, just like the previous one.

<form action= "#" >
<select id= "NAV" >
<option value= "" >Month</option>
<option value= "0" >January</option>
<option value= "1" >February</option>
<option value= "2" >March</option>
</select>
</form>

Window.onload=initform;
function initform () {
document.getElementById ("Nav"). onchange = GetId;
}
function GetId () {
Console.log (This)
var xid= this.options[this.selectedindex].value;
alert (XID);
}//This can get the ID value

This is also possible to get results.

A few examples of this mechanism inside JavaScript