实例
//构造函数
var arr = new Array();
l(arr);
//字面量
var arr1 = [];
l(arr1);数组的length不是只读，可以从数组的末尾移除项或向数组添加新项
var colors = [‘red‘, ‘blue‘, ‘green‘];
l(colors);//(3) ["red", "blue", "green"]
colors.length = 2;
l(colors);//(2) ["red", "blue"]5.2.1 检测数组 (instanceof)
