Introduction to internal attributes and delete operator in js_javascript skills

Before explaining Configurable, let’s first look at an interview question:

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

From the above question, we can see two differences: when a variable is not declared using var, it can be deleted using the delete keyword, and the value will be undefined when obtained again; when a variable is declared using var, It cannot be deleted using delete, and the value is still 2 when retrieved.

1. delete operator

When you use delete to delete a variable or attribute, it returns true if the deletion is successful, otherwise it returns false. As in the above example, if delete cannot delete variable a, it will return false; if delete can successfully delete variable b, it will return true.

In addition to the above two situations, there are various other commonly used variables that can be deleted by delete, and some that cannot be deleted. Let’s not worry about why delete produces such a result. Here we only look at its return value:

Delete an element in the delete array:

// 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]

Delete function type variables:

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

Delete function.length, which is the number of formal parameters obtained:

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

Delete commonly used variables:

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

Delete the prototype instead of deleting the attributes on the prototype:

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

When deleting the length of arrays and strings:

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

When deleting attributes in obj:

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 }

When deleting attributes in an instance object, you can see from the following output that when you use delete to delete attributes, only the attributes of the instance object itself are deleted, and the attributes on the prototype cannot be deleted. Even if you delete it again, it will still be deleted. It cannot be deleted; if you want to delete the attributes or methods of the attributes on the prototype, you can only delete them: delete Person.prototype.name:

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

2. Internal properties of js

In the above example, some variables or attributes can be deleted successfully, while others cannot be deleted. So what determines whether this variable or attribute can be deleted.

ECMA-262 5th Edition defines the characteristics of JS object properties (used in JS engines, not directly accessible from the outside). There are two kinds of properties in ECMAScript: data properties and accessor properties.

2.1 Data attributes

A data attribute refers to a location containing a data value at which the value can be read or written. This attribute has 4 properties that describe its behavior:

• [[configurable]]: Indicates whether it can be deleted and redefined using the delete operator, or whether it can be modified as an accessor attribute. Default is true;
• [[Enumberable]]: Indicates whether the attribute can be returned through a for-in loop. Default true;
• [[Writable]]: Indicates whether the value of the attribute can be modified. Default true;
• [[Value]]: Contains the data value of this attribute. This value is read/written. The default is undefined; for example, the name attribute is defined in the instance object Person above, and its value is 'wenzi'. Modifications to this value are anyway at this location

To modify the default characteristics of object properties (the default is true), you can call the Object.defineProperty() method, which receives three parameters: the object where the property is located, the property name and a descriptor object (must be: configurable, enumberable, writable and value, one or more values ​​can be set).

is as follows:

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

It can be seen that neither delete nor reset the value of person.name takes effect. This is because calling the defineProperty function modifies the characteristics of the object's properties; it is worth noting that once configurable is set to false, defineProperty can no longer be used to Modify it to true (execution will report an error: Uncaught TypeError: Cannot redefine property: name);

2.2 Accessor properties

It mainly includes a pair of getter and setter functions. When reading the accessor attribute, the getter will be called to return a valid value; when the accessor attribute is written, the setter will be called to write the new value; this attribute has the following 4 characteristics :

• [[Configurable]]: Whether the attribute can be deleted and redefined through the delete operator;
• [[Numberable]]: Whether this attribute can be found through a for-in loop;
• [[Get]]: Automatically called when reading properties, default: undefined;
• [[Set]]: Automatically called when writing attributes, default: undefined;

Accessor properties cannot be defined directly and must be defined using defineProperty(), as follows:

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

However, there is still one thing that needs extra attention. When setting properties with the Object.defineProperty() method, accessor properties (set and get) and data properties (writable or value) cannot be declared at the same time. This means that if a property has a writable or value attribute set, then this property cannot declare get or set, and vice versa.

If defined as follows, accessor properties and data properties exist at the same time:

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

上面的代码看起来貌似是没有什么问题，但是真正执行时会报错，报错如下：

Uncaught TypeError: Invalid property. A property cannot both have accessors and be writable or have a value

对于数据属性，可以取得：configurable,enumberable,writable和value；

对于访问器属性，可以取得：configurable,enumberable,get和set。

由此我们可知：一个变量或属性是否可以被删除，是由其内部属性Configurable进行控制的，若Configurable为true，则该变量或属性可以被删除，否则不能被删除。

可是我们应该怎么获取这个Configurable值呢，总不能用delete试试能不能删除吧。有办法滴！！

2.3 获取内部属性

ES5为我们提供了Object.getOwnPropertyDescriptor(object, property)来获取内部属性。

如：

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}

通过Object.getOwnPropertyDescriptor(object, property)我们能够获取到4个内部属性，configurable控制着变量或属性是否可被删除。这个例子中，person.name的configurable是true，则说明是可以被删除的：

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

我们再回到最开始的那个面试题：

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

跟我们使用delete操作删除变量时产生的结果是一样的。

3. 总结

别看一个简简单单的delete操作，里面其实包含了很多的原理！