Undefined==null triggers the difference and connection between the two and the connection page 2/3

What happens in javascript: null   undefined  " "

File 1: null_undefined.html
File 2: null_undefined.js
1
null_undefined.html
【See article comment 1】

null_undefined.js

   var test = function()
{
alert(window.aiter2008);
alt(("test_text1111"));
var testText = ("test_text").value;       // No element was entered in the test_text input box
         alert(testText=="");                                          //true
         alert(testText==null);                                        //false
}

This test can be concluded:
There is no specified variable in an object, but to use it, undefined will appear
The result of elements not found in Html is: null
Some elements in Html, but do not have any value. The result of searching through ("") is: " "; not null
2     null_undefined.html file remains unchanged
null_undefined.js
   var test = function()
{
         var testText = ("test_text");
         var testName = ;
         var testUn = testText.name11;
     alert("test_text's name::"+testName);      // test_text's name:: test_text
         alert("test_text's name11:::"+testUn);       // test_text's name11:::undefined
}                                             
in conclusion
An object does not have an attribute, the call will appear: undefied  This is the same as calling the aiter2008 attribute of the window