js创建数组、读取数组值和设置数组的值

var arr=new Array(4); //创建一个包含3项的空数组
var arr2=new Array('red','blue','yellow'); //['red','blue','yellow']

var arr3=[]; //[]
var arr4=['red','yellow']; //['red','yellow']
var arr5=[1,2,]; //[1,2,]

var arr4=['red','yellow'];
console.log(
  arr4[0],
  arr4[2],
);
//red
//undefined
var arr4=['red','yellow'];
arr4.length=1;
console.log(arr4);
arr4.length=3;
console.log(arr4);
//["red"] 长度变为1
//["red", empty × 2] 长度变为3，后两项为空

  var arr=[1,2,3];
  arr[10]=10;
  console.log(arr);
  //[1, 2, 3, empty × 7, 10] length为10中间的值为undefined