For compressing js files we need yuicompressor-2.4.7.jar file . Add below metioned target in your build file and then call it as needed.
<target name="compressingjsfile"> <echo message="Compressing Javascript files at location: <<<<Your js folder location>>>>" /> <java jar="lib/yuicompressor-2.4.7.jar" fork="true"> <arg value="yourjsfilename11111.js" /> <!-- input path for JS files --> <arg value="yourjsfilename111222.js" /> <!-- input path for JS files --> <!--<arg value="-v" /> --><!-- Turn on verbose --> <arg value="-o" /> <arg value="'.js$:.js'" /> <arg value="yourjsfilename11111_min_out.js" /> <!-- output path for JS files --> <arg value="yourjsfilename111222_min_out.js" /> <!-- output path for JS files --> <classpath> <pathelement location="lib/yuicompressor-2.4.7.jar"/> </classpath> </java> </target>