document.getElementById introduction_javascript skills

Think of your brain as a browser and execute the following code twice, IE6 and IE9 respectively:

Copy the code The code is as follows:

function testFunc(){
alert('test')
}
$(function(){
var g = document.getElementById ,
w = window.testFunc;
//g
alert(typeof(g));
alert(String(g));
alert(g instanceof Object);
alert(g instanceof Function );
//w
alert(typeof(w));
alert(String(w));
alert(w instanceof Object);
alert(w instanceof Function);
//Execute
alert(g('t'));
w();
});

in standard browsers (IE9, FF , chrome, etc.) The above code is executed very consistently, and the return result is as follows:
typeof => "function"

Copy code The code is as follows:

String => "function #funcName#{[native code]}"
instanceof Object => true
instanceof Function => true

It’s strange that although the type is a function, we cannot directly use brackets to execute function g, but need to use call

g.call(document,elementId);
But if The running environment is IE6, everything looks very weird, the following is the running result (note the bold part):

Copy the code The code is as follows :

//g
typeof => "object"
String => "function getElementById{[native code]}"
instanceof Object => false
instanceof Function => false
//w
typeof => "function"
String => "function testFunc{alert('test')}"
instanceof Object => ; true
instanceof Function => true

Under IE 6, for both g and w, you can only use brackets to directly execute the function without using call. Using the following method to call function g will result in an error "the object does not have this attribute":
g.call(document,eleId)
Under IE6, there is no problem with the test results of the custom function testFunc. But it’s very strange for g!

Since g is an object, why can it be directly called and executed using () like a function?
In standard browsers, since g is a function, why can’t it be executed directly using ()?
In fact, for document.getElementById, whether it is a function or an object, even jQuery 1.6.2 does not solve this problem.
$.isFunction(g) still returns false in IE6! The following is the relevant source code of jQuery.isFunction of jQuery 1.6.2:

Copy code The code is as follows:

class2type={};
...
// Populate the class2type map
jQuery.each ("Boolean Number String Function Array Date RegExp Object".split(" "), function(i, name) {
class2type[ "[object " name "]" ] = name.toLowerCase();
} );
...
type: function( obj ) {
return obj == null ?
String( obj ) :
class2type[ Object.prototype.toString.call(obj) ] || "object";
},
...
isFunction: function( obj ) {
return jQuery.type(obj) === "function";
}

So I raised this question on StackOverflow. Fortunately, there are many good people, and they responded quickly. Finally, I will briefly summarize it for your reference:
document.getElementById was originally defined as a member of the HTMLDocument (HTML DOM) interface, but was later moved to the Document (XML DOM) interface in the level 2 DOM.
document.getElementById belongs to the host object, it is a function, but it is not defined in ECMAScript but is part of the DOM interface.
Support [[Call]] (internal attribute?) The return value of typeof of host object is function. Please remember that Host Objects do not always follow the rules related to Native Objects, such as typeof.
For testFunc it is a native object, more specifically a native function.
The following is the classification of the return results of the typeof operator in EcmaScript 5:

TABLE>So if you want to use $ instead of document.getElementById you need to do this: Copy code The code is as follows: var $ = function(id) { return document.getElementById(g) }; But even after the above explanation, I still have questions about Host Object and Native Object. There are new doubts.

Type of val	Result
Undefined	<strong>"undefined"</strong>
Null	<strong>"object"</strong>
Boolean	<strong>"boolean"</strong>
Number	<strong>"number"</strong>
String	<strong>"string"</strong>
Object (native and does not implement [[Call]])	<strong>"object"</strong>
Object (native or host and does implement [[Call]])	<strong>"function"</strong>
Object (host and does not implement [[Call]])	Implementation-defined except may not be <strong>"undefined"</strong>, <strong>"boolean"</strong>, <strong>"number</strong>", or<strong> "string".</strong>

Type of val

Result

Undefined

<strong>"undefined"</strong>

Null

<strong>"object"</strong>

Boolean

<strong>"boolean"</strong>

Number

<strong>"number"</strong>

String

<strong>"string"</strong>

Object (native and does not implement [[Call]])

<strong>"object"</strong>

Object (native or host and does implement [[Call]])

<strong>"function"</strong>

Object (host and does not implement [[Call]])

Implementation-defined except may not be <strong>"undefined"</strong>, <strong>"boolean"</strong>, "number", or<strong> "string".</strong>