js DOM element ID is the global variable

js DOM element ID is the global variable _DOM

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Release： 2016-05-16 17:49:48

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Standard specifications

HTML5 specification document points out: If an element meets either of the following two rules, then the window object must have a corresponding attribute, and the attribute value is this object .

If an element has an ID attribute, the attribute value of the ID attribute will become the attribute name of the window object.
If an element has a name attribute, then the attribute value of the name attribute will become the attribute name of the window object. But the tag name of this element must be: a, applet, area, embed, form, frame, frameset, iframe, img, object, one of them.

Let's look at an example. Suppose there is a page that contains a div element with the ID attribute "foo":

Copy code The code is as follows:

In this way, the above div element can be accessed through window.foo (like other window properties), or the global variable foo. For example, in the Chrome console, you can do this:

Copy code The code is as follows:

> "foo" in window 
true 
> ; foo 

Firefox

Firefox(14) Works slightly differently.

Copy code The code is as follows:

 
> "foo" in window 
false 
> typeof foo // Does this global variable exist? 
object 
//The error console output the following warning.//Element referenced by ID/NAME in the global scope. 
//Use W3C standard document.getElementById() instead. 

> foo 
[object HTMLDivElement] 
//The error console output the following warning.// Element referenced by ID/NAME in the global scope. 
//Use W3C standard document.getElementById() instead.> "foo" in window true 

Copy code The code is as follows:

> "foo" in window
false
> typeof foo / / Does this global variable exist?
object
//The error console output the following warning.//Element referenced by ID/NAME in the global scope.
//Use W3C standard document. getElementById() instead.

> foo
[object HTMLDivElement]
//The error console output the following warning.//Element referenced by ID/NAME in the global scope.
//Use W3C standard document. getElementById() instead.> "foo" in window true

What is going on? When initialized, window does not have the attribute foo. But when this attribute is accessed for the first time (either directly through the window.foo attribute or through the global variable foo), it will be automatically created.

Translator's Note: I did not find warnings in Firefox14, 15, and 18, but there was indeed a warning when testing Firefox12.

js DOM element ID is the global variable _DOM

[Note: The code in the example can only be effective when run through the script tag in the web page and cannot be run through the terminal. This is because the terminal uses different methods when handling global objects.]

Translator’s Note: I tried the code in the example in Firebug and didn’t find any difference.

Once you try to read the value of foo, although the div element will be returned normally, there will be a warning in the error console telling you that you should not do that. Obviously, such a warning is correct : You can use this feature when debugging in the terminal, but in actual code, you should not use it.

Cody Lindley wrote a jsPerf test to compare accessing foo via global variables with accessing foowindow.foo. Interestingly, only accessing window.foo in Firefox is faster.