I rewrote the code and it works now as intended:
// Adds javaScript to <head>
public function addJavaScript($javascript_file) {
$this->javascript[] = $javascript_file;
}
// Construct
public function __construct() {
// JS
$this->addJavaScript('jquery.js');
$this->addJavaScript('bootstrap.js');
$this->addJavaScript('functions.js');
}
// Adds javaScript (Put this in head)
foreach ($this->javascript as $js_file) {
$html .= '
<script src="js/' . $js_file . '"></script>';
}