VAST tracking events

I am looking into VAST advertisement and I understand the implementation except on thing, there are the tracking URI’s that has to be executed, how exactly it is done with javascript, I could not find any reference about it for example below is an excerpt from a VAST XML file

          <Tracking event="midpoint"><![CDATA[]]></Tracking>
          <Tracking event="complete"><![CDATA[]]></Tracking>
          <Tracking event="start"><![CDATA[]]></Tracking>
          <Tracking event="firstQuartile"><![CDATA[]]></Tracking>
          <Tracking event="close"><![CDATA[]]></Tracking>
          <Tracking event="thirdQuartile"><![CDATA[]]></Tracking>
          <Tracking event="progress" offset="00:00:30.000"><![CDATA[]]></Tracking>
          <Tracking event="progress" offset="60%"><![CDATA[]]></Tracking>

How exactly for example this URI “” is executed with javascript. Any help is appreciated.

Thank you.