ES6 方法覆盖

'use strict' ;
class PrinterClass {
   doPrint() {
      console.log("doPrint() from Parent called… ");
   }
}
class StringPrinter extends PrinterClass {
   doPrint() {
      console.log("doPrint() is printing a string…");
   }
}
var obj = new StringPrinter();
obj.doPrint();