JAVASCRIPT

linking a script .js

<script type="text/javascript" src="path-to-javascript-file.js"></script>

javascript include js file

function loadScript( url, callback ) {
  var script = document.createElement( "script" )
  	  script.type = "text/javascript";
      script.src = url;
 	  script.onload = function() {
     	callback()
      };
     document.head.appendChild(script); 
}
// call the function...
loadScript("js/myscript.js", function() {
  alert('script ready!'); 
});

how to include script file in javascript

import('your-file-name.js')
//this only works for javascript files
//and with bugs or half bugs it won't work

how to include script file in javascript with javascript

check this link:
https://stackoverflow.com/questions/950087/how-do-i-include-a-javascript-file-in-another-javascript-file
it wil tell you it in all js variants (including jQuery and node.js)