[Solved] What is the zero index in getElementsByTagName?

by 


var js = document.createElement('script');
js.src="https://stackoverflow.com/questions/45500080/myscript.js";

document.getElementsByTagName('head')[0].appendChild(js);

You get all head elements (there should be only one) and you add the script there so the result is

<html>
  <head>
    <script>
    ...

if there’s no head in the document, most browsers will create the head element even if the tag is not there.

Have a look at this, it might can help.
http://www.jspatterns.com/the-ridiculous-case-of-adding-a-script-element/

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solved What is the zero index in getElementsByTagName?