js创建及操作数组

var colors = new Array();
创建length值为20的的数组： var colors = new Array(20);
直接传递数组的项： var colors = new Array("red", "blue", "green");

 var colors = ["red", "blue", "green"];

colors[2] = "black"   //修改第三项

colors[3] = "brown"  //新增第四项

colors.length = 2;   //数组长度缩短为2

colors[colors.length] = "white" //设置最后一项

if(value instanceof Array){

　　//执行某些操作

}

if(Array.isArray(value)){

  //对数组执行某些操作(ie9+)

}

var colors = ["red", "blue", "green"];
 alert(colors.toString());   //red,blue,green以逗号分隔的字符串
 alert(colors.valueOf());    //red,blue,green返回的是数组
 alert(colors);              //red,blue,green

var colors2 = ["red", "blue", "green"];
 alert(colors2.join(","));    //red,blue,green
 alert(colors2.join("||"));   //red||blue||green

var colors = new Array();
var count = colors.push("red","blue","green");  //向后插入两项
alert(count);            //3
alert(colors);           //red,blue,green

var item = colors.pop();                //移除最后一项并返回
alert(item);            //green
alert(colors.length)    //2

var item2 = colors.shift();             //移除第一项并返回
alert(item2);           //red
alert(colors.length)    //1

var count2 = colors.unshift("black","white");
alert(count2);          //3
alert(colors);          //black,white,blue

var values = [0,1,5,10,15];
values.reverse();       //翻转数组
alert(values);          //15,10,5,1,0
values.sort();          //升序排列，先调用tostring（）将数组项转换成字符串，然后比较字符串。
alert(values);          //0,1,10,15,5

values.sort(compare);   //加入比较函数
alert(values)           //0,1,5,10,15

function compare(value1,value2){
    return value1 - value1;   //降序
}

//concat()：连接数组产生新的数组
var colors = ["red","blue","green"];
var colors2 = colors.concat("white",["black","yellow"]);
alert(colors);          //"red","blue","green"
alert(colors2);         //"red","blue","green","white","black","yellow"

//slice(start,[end])//基于当前数组产生一个新的数组
var colors = ["red","blue","green","black"];
var colors2 = colors.slice(1);
var colors3 = colors.slice(1,3);
alert(colors);//"red","blue","green","black"
alert(colors2);//"blue","green","black" 没有第二个参数则截取到数组末尾
alert(colors3);//"blue","green"  不包括end位置的项

//splice()删除，插入，替换
var colors = ["red","blue","green","black"];
var removed = colors.splice(0,1);//从下标为0开始，截取一项
alert(removed);//"red"
alert(colors);//"blue","green","black"
removed = colors.splice(1,0,"white","orange");//从下标为1开始，截取0个，并在当前位置插入"white","orange"。
alert(removed);//空，这里没有截取项
alert(colors);//"blue","white","orange","green","black"
removed = colors.splice(1,1,"red","purple");////从下标为1开始，截取1个，并在当前位置插入"white","orange"。
alert(removed);//""white
alert(colors);//"blue","red","purple","orange","green","black"

//indexOf(项,[起点的索引]),lastIndexOf(项,[起点的索引])。要找到的项必须严格相等
var numbers = [1,2,3,4,5,4,3,2,1]; //下标从0到1，不管是从先往后还是从后往前，数组下标不变
alert(numbers.indexOf(4));        //3
alert(numbers.lastIndexOf(4));    //5
alert(numbers.indexOf(4,4));      //5
alert(numbers.lastIndexOf(4,4));  //3

 var numbers = [1,2,3,4,5,6];
 var everyResult = numbers.every(function(item,index,array){
 return (item > 2);          //如果所有item都满足条件，才会返回true
 });
 alert(everyResult);         //false

 var someResult = numbers.some(function(item,index,array){
 return(item>2);             //如果有一个item满足条件，会返回true
 });
 alert(someResult);          //true

 var filterResult = numbers.filter(function(item,index,array){
 return (item > 2);          //截取满足条件的item并返回
 });
 alert(filterResult);       //3,4,5,6

 var mapResult = numbers.map(function(item,index,array){
 return (item * 2);          //对所有item操作并返回
 });
 alert(mapResult);//2,4,6,8,10,12

 var numbers = [1,2,3,4,5,6];
 var sum = numbers.reduce(function(prev,cur,index,array){
 return prev + cur;//返回的值会作为下一个prev继续操作，最终返回的是所有项相加的值.从左往右
 });
 alert(sum);//21
 var sum2 = numbers.reduceRight(function(prev,cur,index,array){
 return prev + cur;//返回的值会作为下一个prev继续操作，最终返回的是所有项相加的值.从右往左
 });
 alert(sum2);//21