'JavaScript Advanced Programming' study notes object and array reference types_javascript skills

This article will share with you my JavaScript advanced programming study notes on object and array reference types, which involve knowledge about JavaScript reference types. Let’s take a look.

1. Object type

Most reference type values ​​are instances of the Object type; and Object is also the most commonly used type in ECMAScript.

There are two ways to create an Object instance:

new operator followed by Object constructor:

   var person=new Object( );
   person.name="webb";
   person.age=25;

Object literal notation:

  var person={
     name:"webb",
     age:25
   };

2. Array type

In addition to Object, the Array type is probably the most commonly used type in ECMAScript.
Each item of an ECMAScript array can hold any type of data (for example, a string can be stored in the first position, a value in the second position, an object in the third position, and so on). Moreover, the size of the ECMAScript array can be dynamically adjusted, that is, it can automatically grow as data is added to accommodate the new data.

There are two basic ways to create an array,

Use Array constructor:

var colors=new Array( );
     var colors=new Array(20);   //也可设置length属性
     var colors=new Array("red","blue","green");   //包含3个字符串的数组
     var colors=Array(3);   //可省略new操作符

Array literal notation

 var colors=["red","blue","green"];
     alert(colors[0]);   //显示第一项
     colors[2]="black";   //修改第三项
     colors[3]="brown";   //新增第四项

Note: The length property of the array is very special - it is not read-only. So by setting this property, items can be removed or added from the end of the array. For example,

     var colors=["red","blue","green"];
     colors.length=2;
     alert(colors[2]);   //undefined
     colors[colors.length]="black";   //在末尾添加项

 2.1 Detection Array

For a web page or a global scope, you can use the instanceof operator to determine whether an object is an array:

   if(value instanceof Array){
        //对数组执行某些操作
     }

The problem with the instanceof operator is that it assumes there is only one global execution environment. If the web page contains multiple frames, there are actually more than two different global execution environments, and thus more than two different versions of the Array constructor. If you pass an array from one frame to another, the array you pass in will have a different constructor than the array created natively in the second frame.

To solve this problem, ECMAScript5 added the Array.isArray() method. The purpose of this method is to ultimately determine whether a value is an array, regardless of the global execution environment in which it was created,

if(Array.isArray(value)){
        //对数组执行某些操作
     }

Browsers that support this method include IE9, Firefox 4, Safari 5, Opera 10.5 and Chrome.

2.2 Conversion method

Calling the toString() method of an array will return a comma-delimited string concatenated from the string form of each value in the array. And calling valueOf() still returns an array. In fact, to create this string the toString() method of each item in the array is called. For example,

var colors=["red","blue","green"];
     alert(colors.toString());   //red,blue,green
     alert(colors.valueOf());   //red,blue,green
     alert(colors);   //red,blue,green

In addition, the toLocaleString() method often returns the same value as the toString() and valueOf() methods, but this is not always the case. When the toLocaleString() method of an array is called, it also creates a comma-delimited string of the array values. The only difference from the first two methods is that this time, in order to obtain the value of each item, the toLocaleString() method of each item is called instead of the toString() method.

  var person1={
        toLocaleString:function(){
          return "webbxx";
        },
        toString:function(){
          return "webb";
        }
     };
     var person2={
        toLocaleString:function(){
          return "susanxx";
        },
        toString:function(){
          return "susan";
        }
     };
     var people=[person1,person2];
     alert(people);   //webb,susan
     alert(people.toString());   //webb,susan
     alert(people.toLocaleString());   //webbxx,susanxx
     使用join( )方法也可输出数组，并可指定分隔符，默认为逗号：
          var colors=["red","blue","green"];
          alert(colors.join(","));   //red,blue,green
          alert(colors.join("||"));   //red||blue||green

2.3 Stack method (LIFO)

push(): accepts any number of parameters, adds them to the end of the array one by one, and returns the length of the modified array;
             pop(): Remove the last item from the end of the array

  var colors=new Array();
        var count=colors.push("red","green");
        alert(count);   //2
        count=colors.push("black");
        alert(count);   //3
        var item=colors.pop();
        alert(item);   //"black"
        alert(colors.length);   //2

2.4 队列方法(FIFO)

shift( )：移除数组的第一项并返回该项，同时数组长度减1；
unshift( )：顾名思义，与shift( )用途相反，能在数组前端添加任意个项并返回数组的长度。

2.5 重排序方法

reverse( )：反转数组项的顺序；

sort( )：默认按升序排列；为了实现排序，sort( )方法会调用每项的toString( )方法，然后比较得到的字符串，以确定如何排序。即使每一项都是数值，比较的也是字符串，如下所示。

   var values=[0,1,5,10,15];
        values.sort();
        alert(values);   //0,1,10,15,5

这种排序方式在很多情况下都不是最佳方案。因此sort( )方法可以接受一个比较函数作为参数，以便指定哪个值位于哪个值的前面。

 function compare(value1,value2){
        if(value1<value2){
          return -1;   //value1在value2之前
        }else if(value1>value2){
          return 1;
        }else{
          return 0; 
        }
     }

这个比较函数可以适用大多数据类型，只要将其作为参数传递给sort( )方法即可，如下，

 var values=[0,1,5,10,15];
        values.sort(compare);
        alert(values);   //0,1,5,10,15

2.6 操作方法

concat( )：基于当前数组中的所有项创建一个新数组。例如，

 var colors=["red","blue","green"];
        var colors2=colors.concat("yellow",["black","brown"]);
        alert(colors);   //red,blue,green
        alert(colors2);   //red,blue,green,yellow,black,brown

slice( )：基于当前数组中的一个或多个项创建一个新数组。例如，

       var colors=["red","green","blue","yellow","purple"];
        var colors2=colors.slice(1);   //green,blue,yellow,purple
        var colors3=colors.slice(1,3);   //green,blue,yellow

splice( )：这个方法恐怕是最强大的数组方法了，主要用途是向数组的中部插入项，但使用这种方法的方式则有如下2种。

删除：可以删除任意数量的项，只需指定2个参数：要删除的第一项的位置和要删除的项数。

插入：可以向指定位置插入任意数量的项，只需提供3个参数：起始位置、0（要删除的项数）和要插入的项。如果要插入多个项，可以再传第四、第五以至任意多个项；例如，splice(2,0,"red","green")会从当前数组的位置2开始插入字符串"red"和"green"。

splice( )方法始终都会返回一个数组，包含从原始数组中删除的项（如果没有删除任何项，则返回空数组）。

2.7 位置方法

indexOf( )和lastIndexOf( )：这两个方法都接受两个参数：要查找的项和（可选的）表示查找起点位置的索引。前者从开头开始向后查找，后者从末尾向前查找

2.8 迭代方法

ECMAScript5为数组定义了5个迭代方法，每个方法都接受两个参数：要在每一项上运行的函数和（可选的）运行该函数的作用域对象——影响this的值。传入这些方法中的函数会接收三个参数：数组项的值、该项在数组中的位置和数组对象本身。

every( )：对数组中的每一项运行给定函数，如果该函数对每一项都返回true，则返回true。
filter( )：对数组中的每一项运行给定函数，返回该函数会返回true的项组成的数组。
forEach( )：对数组中的每一项运行给定函数，这个方法无返回值。
map( )：对数组中的每一项运行给定函数，返回每次函数调用的结果组成的数组。
some( )：对数组中的每一项运行给定函数，如果该函数对任一项返回true，则返回true。

以上方法都不会修改数组中包含的值。例如，

var numbers=[1,2,3,4,5,4,3,2,1];
        var everyResult=numbers.every(function(item,index,array){
          return item>2;
        });
        alert(everyResult);   //false
        var someResult=numbers.every(function(item,index,array){
          return item>2;
        });
        alert(someResult);   //true
        var filterResult=numbers.every(function(item,index,array){
          return item>2;
        });
        alert(filterResult);   //[3,4,5,4,3]
        var mapResult=numbers.every(function(item,index,array){
          return item*2;
        });
        alert(mapResult);   //[2,4,6,8,10,8,6,4,2]

2.9 归并方法

reduce( )：从数组的第一项开始，逐个遍历到最后；
reduceRight( )：从数组的最后一项开始，向前遍历到第一项。
这两个方法都接受两个参数：一个在每一项上调用的函数和（可选的）作为归并基础的初始值。传给这些方法的函数接受4个参数：前一个值、当前值、项的索引和数组对象。这个函数返回的任何值都会作为第一个参数传给下一项。例如，

   var values=[1,2,3,4,5];
        var sum=values.reduce(function(prev,cur,index,array){
          return prev+cur;
        });
        alert(sum);   //15