a = 1;
console.log( window.a ); // 1
console.log( delete window.a ); // true
console.log( window.a ); // undefined

var b = 2;
console.log( window.b ); // 2
console.log( delete window.b ); // false
console.log( window.b ); // 2

// 使用for~in是循环不到的，直接忽略到该元素
// 使用for()可以得到该元素，但是值是undefined
var arr = [1, 2, 3, 4];
console.log( arr );      // [1, 2, 3, 4]
console.log( delete arr[2] ); // true，删除成功
console.log( arr );       // [1, 2, undefined, 4]

// chrome 不能删除；火狐可以删除
function func(){
}
console.log( func );
console.log( delete func );
console.log( func );

function func1(a, b){
}
console.log( func1.length );    // 2
console.log( delete func1.length ); // true，删除成功
console.log( func1.length );    // 0

console.log( delete NaN );   // false，删除失败
console.log( delete undefined );// false
console.log( delete Infinity ); // false
console.log( delete null );   // true，删除成功

function Person(){
}
Person.prototype.name = "蚊子";
console.log( delete Person.prototype ); // false，无法删除
console.log( delete Object.prototype ); // false

var arr = [1, 2, 3, 4];
console.log( arr.length );     // 4
console.log( delete arr.length );  // false，删除失败
console.log( arr.length );     // 4

var str = 'abcdefg';
console.log( str.length );     // 7
console.log( delete str.length );  // false，删除失败
console.log( str.length );     // 7

var obj = {name:'wenzi', age:25};
console.log( obj.name );    // wenzi
console.log( delete obj.name ); // true，删除成功
console.log( obj.name );    // undefined
console.log( obj );       // { age:25 }

function Person(){
  this.name = 'wenzi';
}
Person.prototype.name = '蚊子';
var student = new Person();
console.log( student.name );    // wenzi
console.log( delete student.name ); // true，删除成功
console.log( student.name );    // 蚊子
console.log( delete student.name ); // true
console.log( student.name );    // 蚊子
console.log( delete Person.prototype.name );// true，删除成功
console.log( student.name );    // undefined

var person = {};
Object.defineProperty(person, 'name', {
  configurable: false, // 不可删除，且不能修改为访问器属性
  writable: false,  // 不可修改
  value: 'wenzi'   // name的值为wenzi
});
console.log( person.name);   // wenzi
console.log( delete person.name ); // false，无法删除
person.name = 'lily';
console.log( person.name );   // wenzi

var person = {
  _age: 18
};
Object.defineProperty(person, 'isAdult', {
 Configurable : false,
  get: function () {
    if (this._age >= 18) {
      return true;
    } else {
      return false;
    }
  }
});
console.log( person.isAdult ); // true

var o = {};
Object.defineProperty(o, 'name', {
  value: 'wenzi',
  set: function(name) {
    myName = name;
  },
  get: function() {
    return myName;
  }
});

var person = {name:'wenzi'};
var desp = Object.getOwnPropertyDescriptor(person, 'name'); // person中的name属性
console.log( desp ); // {value: "wenzi", writable: true, enumerable: true, configurable: true}

console.log( person.name );   // wenzi
console.log( delete person.name ); // true，删除成功
console.log( person.name );   // undefined

a = 1;
var desp = Object.getOwnPropertyDescriptor(window, 'a');
console.log( desp.configurable );  // true，可以删除

var b = 2;
var desp = Object.getOwnPropertyDescriptor(window, 'b');
console.log( desp.configurable );  // false，不能删除