function Person( uName ){
  if ( this instanceof Person ) {
   this.userName = uName;
  }else {
   return new Person( uName );
  }
 }
 Person.prototype.showUserName = function(){
  return this.userName;
 }
 console.log( Person( 'ghostwu' ).showUserName() );
 console.log( new Person( 'ghostwu' ).showUserName() );

function Person( uName ){
   if( new.target !== undefined ){
    this.userName = uName;
   }else {
    throw new Error( '必须用new实例化' );
   }
  }
  // Person( 'ghostwu' ); //报错
  console.log( new Person( 'ghostwu' ).userName ); //ghostwu

function Person( uName ){
   if ( new.target === Person ) {
    this.userName = uName;
   }else {
    throw new Error( '必须用new实例化' );
   }
  }
  
  // Person( 'ghostwu' ); //报错
  console.log( new Person( 'ghostwu' ).userName ); //ghostwu

class Person{
   constructor( uName ){
    if ( new.target === Person ) {
     this.userName = uName;
    }else {
     throw new Error( '必须要用new关键字' );
    }
   }   
  }

  // Person( 'ghostwu' ); //报错
  console.log( new Person( 'ghostwu' ).userName ); //ghostwu

let Person = ( function(){
   'use strict';
   const Person = function( uName ){
    if ( new.target !== undefined ){
     this.userName = uName;
    }else {
     throw new Error( '必须使用new关键字' );
    }
   }

   Object.defineProperty( Person.prototype, 'sayName', {
    value : function(){
     if ( typeof new.target !== 'undefined' ) {
      throw new Error( '类里面的方法不能使用new关键字' );
     }
     return this.userName;
    },
    enumerable : false,
    writable : true,
    configurable : true
   } );

   return Person;
  })();

  console.log( new Person( 'ghostwu' ).sayName() );
  console.log( Person( 'ghostwu' ) ); //没有使用new,报错