Question

how to load external JS file into moodle ? using moodle api ,i.e moodle libraries ..

Answer 1

In Moodle 2.0 I have always used $PAGE->requires->js().

To start with, make $PAGE available to your code by doing:

require_once($CFG->libdir . '/pagelib.php');
global $PAGE;

and then in your code:

$PAGE->requires->js( new moodle_url($CFG->wwwroot . '/blocks/your_block/script.js') );

It is required to put moodle_url() around your path!