| Actually there is such a function in PHP which function is "include".
It's very strong function.
But Javascript does not support such a function.
Here's the alternate method to do that.
To include javascript, following is using usually:
<script type="text/javascript" src="the_script.js"></script>
But above script is not available in Javascript so following is the alternate method to include another javascript.
the_another_javascript.js:
document.write( "<script type="text/javascript" src="the_script.js"></script>");
Isn't it simple ?
Actually the main logic above is writing script tag in the document. | 
