var num = [1,2,3];   // 创建一个包含3个数字的数组
num.length = 2;
console.log(num);         // [1,2]

var num = [1, 2, 3];
delete num[0];
console.log(num);   //[undefined, 2, 3]

var num = [1,2, 3];
num.pop();
console.log(num)    // [1,2]

var num = [1,2,3];
num.shift();
console.log(num);   // [2,3]

var num = [1,2,3];
num.splice(0, 1);
console.log(num);    // [2,3]

var num = [1,2,3];

num.forEach(function(item, index, arr) {
    if(item == 2) {
        arr.splice(index, 1);
    }
});
console.log(num)  // [1,3]

var num = [1,2,3];
num = num.filter(function(item) {
   return item != "2"
});
console.log(num);    // [1,3]