Clever usage of Javascript apply and extended use of Object.defineProperty

Math.max implements getting the largest item in the array

var array = [1,2,3,4,5];var max = Math.max.apply(null, array);
console.log(max); // 5

When calling, the first parameter is given as null. This is because there is no object to call this method, only You need to use this method to help with the calculation and get the returned result, so you pass a null directly.

Math.min realizes getting the smallest item in the array

var array = [1,2,3,4,5];var min= Math.min.apply(null, array);console.log(min); // 1

Add the max and min methods on the native object

Then you need to use Go to the native object method Object.defineProperty(), which will directly define a new property on an object, or modify an existing property of an object, and return the object

    Object.defineProperty(Array.prototype, 'max', {  
        writable: false,  
        enumerable: false,  
        configurable: true,  
        value: function () {  return Math.max.apply(null, this);  
        }  
    });  
      
    Object.defineProperty(Array.prototype, 'min', {  
        writable: false,  
        enumerable: false,  
        configurable: true,  
        value: function () {  return Math.min.apply(null, this);  
        }  
    });

Just call it directly on the array:

var arr = [54,545,2165,545,56];  
console.log(arr.max());  
console.log(arr.min());

The method of Object.defineProperty was mentioned above, let’s understand it below.

Usage of Object.defineProperty

An object is an unordered collection composed of multiple name/value pairs. Each property in an object corresponds to a value of any type. Objects can be defined using constructors or literals:

var obj = new Object;  //obj = {}obj.name = "张三";  //添加描述obj.say = function(){};  //添加行为

In addition to the above methods of adding properties, you can also use Object.defineProperty Define new properties or modify existing properties.

Object.defineProperty() description

Syntax: Object.defineProperty(obj, prop, descriptor)

Parameter description :

• obj: Required. Target Object

• #prop: Required. The name of the attribute to be defined or modified

• descriptor: required. Characteristics possessed by the target attribute

Return value:

• The object passed into the function. That is, the first parameter obj

is for the attribute. We can set some characteristics for this attribute, such as whether it is read-only and not writable; whether it can be for..inor Object.keys()Traverse.

Add characteristic description to the object's attributes. Currently, two forms are provided: data description and accessor description.

Data description

When modifying or defining an attribute of an object, add some characteristics to this attribute:

var obj = {
    test:"hello"}//对象已有的属性添加特性描述Object.defineProperty(obj,"test",{
    configurable:true | false,
    enumerable:true | false,
    value:任意类型的值,
    writable:true | false});//对象新添加的属性的特性描述Object.defineProperty(obj,"newKey",{
    configurable:true | false,
    enumerable:true | false,
    value:任意类型的值,
    writable:true | false});

In the data description The attributes are all optional. Let’s take a look at the role of setting each attribute.

value

The value corresponding to the attribute can be any type of value. The default is undefined

var obj = {}//第一种情况：不设置value属性Object.defineProperty(obj,"newKey",{

});
console.log( obj.newKey );  //undefined------------------------------//第二种情况：设置value属性Object.defineProperty(obj,"newKey",{
    value:"hello"});
console.log( obj.newKey );  //hello

writable

attribute Whether the value can be overridden. Set to true and can be overridden; set to false and cannot be overridden. Default is false.

var obj = {}//第一种情况：writable设置为false，不能重写。Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:false});//更改newKey的值obj.newKey = "change value";
console.log( obj.newKey );  //hello//第二种情况：writable设置为true，可以重写Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:true});//更改newKey的值obj.newKey = "change value";
console.log( obj.newKey );  //change value

enumerable

Whether this property can be enumerated (using for...in or Object.keys()). If set to true, it can be enumerated; if set to false, it cannot be enumerated. Default is false.

var obj = {}//第一种情况：enumerable设置为false，不能被枚举。Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:false,
    enumerable:false});//枚举对象的属性for( var attr in obj ){
    console.log( attr );  
}//第二种情况：enumerable设置为true，可以被枚举。Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:false,
    enumerable:true});//枚举对象的属性for( var attr in obj ){
    console.log( attr );  //newKey}

configurable

Whether the target attribute can be deleted or whether the properties of the attribute can be modified again (writable, configurable, enumerable). If set to true, the attribute can be deleted or the attribute can be reset; if set to false, the attribute cannot be deleted or the attribute cannot be reset. Default is false.

This attribute plays two roles:

• Whether the target attribute can be deleted using delete

• Whether the target attribute can be deleted again Setting properties

//-----------------测试目标属性是否能被删除------------------------var obj = {}//第一种情况：configurable设置为false，不能被删除。Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:false,
    enumerable:false,
    configurable:false});//删除属性delete obj.newKey;
console.log( obj.newKey ); //hello//第二种情况：configurable设置为true，可以被删除。Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:false,
    enumerable:false,
    configurable:true});//删除属性delete obj.newKey;
console.log( obj.newKey ); //undefined//-----------------测试是否可以再次修改特性------------------------var obj = {}//第一种情况：configurable设置为false，不能再次修改特性。Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:false,
    enumerable:false,
    configurable:false});//重新修改特性Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:true,
    enumerable:true,
    configurable:true});
console.log( obj.newKey ); //报错：Uncaught TypeError: Cannot redefine property: newKey//第二种情况：configurable设置为true，可以再次修改特性。Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:false,
    enumerable:false,
    configurable:true});//重新修改特性Object.defineProperty(obj,"newKey",{
    value:"hello",
    writable:true,
    enumerable:true,
    configurable:true});
console.log( obj.newKey ); //hello

In addition to setting properties for newly defined properties, you can also set properties for existing properties

//定义对象的时候添加的属性，是可删除、可重写、可枚举的。var obj = {
    test:"hello"}//改写值obj.test = 'change value';

console.log( obj.test ); //'change value'Object.defineProperty(obj,"test",{
    writable:false})//再次改写值obj.test = 'change value again';

console.log( obj.test ); //依然是：'change value'

Tip: Once you use Object.defineProperty to add properties to an object, if you do not set the properties of the property, then configurable, enumerable,writableThese values ​​are the default false

##

var obj = {};//定义的新属性后，这个属性的特性中configurable，enumerable，writable都为默认的值false//这就导致了newkey这个是不能重写、不能枚举、不能再次设置特性//Object.defineProperty(obj,'newKey',{

});//设置值obj.newKey = 'hello';
console.log(obj.newKey);  //undefined//枚举for( var attr in obj ){
    console.log(attr);
}

Feature summary of settings:

• value: Set the value of the attribute

• writable: Whether the value can be rewritten. true | false

• enumerable: Whether the target property can be enumerated. true | false

• configurable: Whether the target property can be deleted or whether the property can be modified again true | false

Accessor Description

When using an accessor to describe the characteristics of an attribute, the following attribute attributes are allowed to be set:

var obj = {};
Object.defineProperty(obj,"newKey",{
    get:function (){} | undefined,
    set:function (value){} | undefined
    configurable: true | falseenumerable: true | false});

Note: When using the getter or setter method, it is not allowed Use the two attributes writable and value

getter/setter

When setting or getting the value of a property of an object, you can provide a getter/setter method.

• getter is a method to get the attribute value

• setter is a method to set the attribute value.

Use get/set attributes in attributes to define corresponding methods.

var obj = {};var initValue = 'hello';
Object.defineProperty(obj,"newKey",{
    get:function (){//当获取值的时候触发的函数return initValue;    
    },
    set:function (value){//当设置值的时候触发的函数,设置的新值通过参数value拿到initValue = value;
    }
});//获取值console.log( obj.newKey );  //hello//设置值obj.newKey = 'change value';

console.log( obj.newKey ); //change value

Note: get or set do not have to appear in pairs, just write either one. If the method is not set, the default value of get and set is undefined

Compatibility

It can only be used on DOM objects under IE8, try to use it on native objects Object.defineProperty() will report an error.

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