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问题:

How can I reach some variable declared in another place from within an external javascript file ?

Suppose in an html file I have the following

<head>
 <script>
   var a = 'something';
 </script>
<head>
<body>
<iframe src="otherfile.html"/>
</body>

and inside otherfile.html, in the section, I have

alert(a);

How can I make sure I get an alert message saying "something" ?

I think Google Adsense does this, their code is:

<script type="text/javascript"><!--
google_ad_client = "youdontneedtoknowthis";
google_ad_slot = "5404192644";
google_ad_width = 728;
google_ad_height = 90;
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>

How can the script in http://pagead2.googlesyndication.com/pagead/show_ads.js know those variables (e.g. google_ad_client) ?

回答1:

when a variable is defined without var it is said to exist in global scope. When your JS is loaded into your page, it is aware of any variables that exist globally at that time. So, what google is doing is defining some variables and then loading in an external script.

回答2:

It doesn't matter.

All <script> blocks in the page share the same context and variables.

回答3:

The script can just assume that those variables will be set. They must be because they are needed. The script can check if they are undefined and fail gracefully (do nothing) when they are not. The variables are not in another namespace.