它是当前正在运行的脚本的最后一个 `script` 元素吗? [英] Is it the last `script` element the currently running script?
问题描述
Is it safe to assume that the last script element* in the document when the script runs** is the currently running script?
For example, I want to create a script that can be dropped anywhere in the body of of a page and display an element in the same place. I'm doing something like this:
function getCurrentScriptElement() {
var scripts = document.getElementsByTagName('script');
return scripts[scripts.length - 1];
}
var script = getCurrentScriptElement();
var view = document.createElement('span');
/* Put stuff in our view... */
script.parentNode.insertBefore(view, script);
Assuming the script is in the body of the document, is this "safe?" Will the getCurrentScriptElement function always return the running script? If not, how can it be done?
I'd like to do this without tying the script to a specific id attribute or similar, I'd like it to just be positional.
I created an example here that pulls in this script. One answer suggested that other scripts could create a condition where an example like this would break. Is it possible to add other scripts to this example that will break it?
It was suggested that other scripts with defer or async attributes could break this. Can anyone give an example of how such a script might work?
As I understand it, defer means load the DOM first, and then run the script with the defer tag. How would the defer attribute appearing on another script element affect the behavior of getCurrentScriptElement?
async, as I understand it, means start fetching that script and keep parsing the DOM at the same time, don't wait... but when it hits my script it should still stop and wait, right?
I don't see how either one could affect it, can anyone provide an example?
* I'm only interested in external scripts for the purpose of this question.
** Not the last script element in the entire document, but the last script element in the document at the time when it runs. The rest of the document shouldn't be loaded yet, right?
It's not an absolute guarantee no. Check out this JSFiddle: http://jsfiddle.net/jAsek/
<!DOCTYPE html>
<title>Test case</title>
<div>
<p>At the start</p>
<script id="first">
var scr1 = document.createElement("script");
scr1.setAttribute("id", "early");
document.body.appendChild(scr1);
</script>
<p>After the first script</p>
<script id="second">
function getCurrentScriptElement() {
var scripts = document.getElementsByTagName('script');
return scripts[scripts.length - 1];
}
alert(getCurrentScriptElement().id);
</script>
<p>At the end</p>
</div>
Here the alert reports the id of the injected script "early", not the id of currently running script "second".
There's no practical difference between internal and external scripts.
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