JS Object-Oriented programming: JS class definition function when prototype and this difference?

In the object-oriented Programming JS script, there are two main ways to define an instance

As follows:

function ListCommon2 (afirst) {   var first=afirst;   This.do1=function () {    alert ("First do" +first);}                  }   Listcommon2.prototype.do2=function () {     //  alert ("First do" +first);//error, no access to Firstthis.do1 ();}  

What is the difference between this.do1=function () and listcommon2.prototype.do2=function ()?

is equivalent to the class instance method, only new can be used, then what is the difference?

Test code:

var t2=new ListCommon2 ("Boiling water 2");     T2.do1 ();//T2.do2 ();//

It has been tested that THIS.DO1 can access the variable first inside the constructor, but ListCommon2.prototype.do2 cannot access it, but can access the function This.do1.

If the LISTCOMMON2.PROTOTYPE.DO2 is defined inside the constructor, it is also accessible. However, as an instance function, if it is defined inside the constructor, each instantiation is executed, obviously wasting memory, is not reasonable.

Some of the information on the This.do1 method is called the privileged method, mainly to access the internal private fields, so that you can control access to some fields. For example, a private field first is defined, and can only be passed through a constructor, and then it cannot be modified.

ListCommon2.prototype.do2 such methods are equivalent to instance methods of a class, but can access these privileged methods and indirectly access private fields.

Conclusion:

If you want to access private fields directly, you should use the privileged method, which is the method defined by this, which should be defined inside the constructor. Conversely, if you do not need direct access to private fields, you should use the method defined by prototype and should be defined outside the constructor.