Javascript 'this' keyword returning href attribute rather than Object on anchor tag

问题

Requirement: Return the element object.
Problem: Using the code below, I expected the links to return [object], but they actually return the string in the href attribute (or in the case of the first link, the Window object).

(The HTML below has been tested in FireFox 3.6.8 and Internet Explorer 7 (7.0.6002.18005) with the same results.)

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
      <title>Anchor onclick tests</title>
      <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    </head>
    <body>
        <div>
            <a href="javascript:alert(this);" title="">&lt;a href=&quot;javascript:alert(this);&quot;&gt;...&lt;a/&gt;</a> - Returns: [object Window]<br />
            <a href="#" onclick="alert(this);" title="">&lt;a href=&quot;#&quot; onclick=&quot;alert(this);&quot;&gt;...&lt;a/&gt;</a> - Returns: Full URI<br />
            <a href="javascript:void(0);" onclick="alert(this);" title="">&lt;a href=&quot;javascript:void(0);&quot; onclick=&quot;alert(this);&quot;&gt;...&lt;a/&gt;</a> - Returns: javascript:void(0);
        </div>
    </body>
</html>

Adding .tagname to the this keyword returns undefined for the first link but correctly identifies the second and third as A. Likewise, requesting .href returns undefined for the first link but correctly outputs the href (full URI in the case of '#').

Does anyone know why, and how I can get a hold on the A object itself?

回答1:

As you said, accessing a property in the second and third link works. That means that this is indeed the A DOM element but when it is converted to a string (which is what happens when you want to alert it) it is converted to the URL.

So you already have your object ;)

Same happens when you do alert(document.location). It is actually an object but when converted to a string, it prints the current location.

回答2:

When alert is invoked, the toString method is internally invoked so for the case of the anchors alerting the href, the toString of anchors returns the href instead.

<a id="foo" href="blah">fsdjl</a>

In the JS console, do:

document.getElementById('foo').toString()

This would confirm it.

As for window, this isn't bound to a method owned by the anchor, so this refers to the global context. onclick is bound to the anchor so this, aka current execution context, changes to the anchor.

<a href="#" onclick="javascript:alert( this.nodeName )">blah</a>

The result is it alerts A which is the nodeName, so the browser does return the href if there is an href set so it's more "readable" and less vague.

来源：https://stackoverflow.com/questions/3363848/javascript-this-keyword-returning-href-attribute-rather-than-object-on-anchor

标签

javascript

object

onclick