接着上次的来。

稀疏数组

从0开始索引不连续的数组，length的长度大于元素的个数

var a1 = new Array(5)var a1 = []a1[1000] = 0var a2 = [,]    //稀疏var a2 = [,,]    //非稀疏，元素的值为undefinedvar a3 = [1,,3]    //非稀疏var a3 = [1, undefined, 3]    //非稀疏var  a = [1, 2, 3, 4], delete a[1]    //稀疏

数据的长度

[].length    //0[1, 2, 3].length    //3

稀疏数组的长度大于元素的个数

var a = [1, 2, 3, 4, 5]a.length = 3    //a为[1, 2, 3]a.length = 0    //a为[]a.length = 5    //a 相当于 new Array(5)Object.defineProperty(a, "length", {writable: false};    //设置a的length属性为只读，适用于ECMAScript5

数组元素的增加和删除

基本：

var a = [];a[0] = 1;a[1] = 2;

push尾部增加:

var a = [];a.push("zero");a.push(1, "5");

unshift头部增加:

a.unshift("0","sss");

对于一个使用delete不会修改数据的length属性，也不会改变数据元素的索引

var a1 = [1, 2, 3]delete a1[1];    // [1, , 3]a1.length    //长度仍是3a1.pop()    //返回3，数组变为[1,]a1.shift()    //返回1，数组变为[ ]。即移除数组头部的一个元素，长度减1,元素索引号下移

数组遍历

for循环

function Pasta(grain, width, shape) {     this.grain = grain;     this.width = width;     this.shape = shape;      // Define a method.     this.toString = function () {         return (this.grain + ", " + this.width + ", " + this.shape);     } } var o = new Pasta("wheat", 0.2, "circle"); // { grain: 'wheat',//  width: 0.2,//  shape: 'circle',//  toString: [Function] }var keys = Object.keys(o); //返回对象的可枚举属性和方法的名称，["grain","width","shape","toString"]var values = [];for (var i = 0, len = keys.length; i < len; i ++) {     var key = keys[i];     values[i] = o[key];    //数组的值为对象o的属性值及办法的值}for ( var i = 0; i < a.length; i ++) {    if (!a[i]) continue;    //跳过null, undefined和不存在的元素    if (a[i] === undefined) continue;    //跳过undefined    if (!(i in a )) continue;    //跳过不存在的元素}

for in 循环

for (var index in sparseArray) {    var value = sparseArray[index];}