JS: use the more efficient lastIndexOf using an integer value
JS: use the more efficient lastIndexOf using an integer value

Signed-off-by: Harvey Harrison <eadbd6b462bf3c97df0300a934c12bc2e5d1fe51@gmail.com>

jaamsim/jaamsim,jaamsim/jaamsim,jaamsim/jaamsim,jaamsim/jaamsim