数组

创建数组

var array = new Array();
var array = [];
var array = [1,6,3];
var array = [
    163,
    "netease",
    {color:"red"},
    [],
    true
];

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];

arr.length 表示数组里面有多少个元素

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students.length;//3
students = [];
students.length;//0

获取数组元素

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students[0];//{id:1,score:80}
students[0].score;//80

修改数组元素

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students[1].score = 60;

arr.indexOf(searchElement[,fromIndex = 0]) 输入一个元素，找出该元素在数组中的索引位置

var telephones = [110,120,114];
telephones.indexOf(120);//1
telephones.indexOf(119);//-1

arr.forEach(callback[,thisArg])

做一个遍历，把数组中所有元素都找一遍；传入一个回调函数，在遍历数组时，每遍历一个元素就会执行传入的方法，并且把遍历到的数据传入这个方法去执行；thisArg对象替代callback里的this，即让this指向thisArg。

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
var editScore = function(item,index,array){
    item.score +=5;
};
students.forEach(editScore);

arr.reverse()

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students.reverse();
students[0].score;//70

arr.sort([compareFunction])

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
var byScore = function(a,b){
    return b.score - a.score; 
};//从大到小，如果小于0，a放在b的前面，如果大于0，a放在b的后面，如果是0，则a、b保持原有顺序
students.sort(byScore);
//直接改变了原来的数组
[
    {id:1,score:80},
    {id:3,score:70},
    {id:2,score:50}
]

//不传回调函数则按万国码unicode顺序排列,先比较第一位再比较第二位
var studentsNames = ["wq","xl","gp"];
studentNames.sort();
studentNames;//["gp","wq","xl"]

arr.push(element1,...,elementN)在已有的数组上面往后再加上一些元素

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students.push({id:4,score:90});
[
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70},
    {id:4,score:90}
]

arr.unshift(element1,...,elementN)在已有的数组上面往前加上一些元素

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students.unshift({id:4,score:90});
[
    {id:4,score:90},
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
]

arr.shift() 获取数组里面第一个元素，并且原来的数组会被删掉第一个元素

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students.shift();//{id:1,score:80}
//原数组
[
    {id:2,score:50},
    {id:3,score:70}
]

arr.pop()获取数组里面最后一个元素，并且原来的数组会被删掉最后一个元素

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students.pop(); //{id:3,score:70}
//原数组
[
    {id:1,score:80},
    {id:2,score:50}
]

在任意位置插入或者删除

arr.splice(index,howMany[,ele1[,...[eleN]]]) 从哪个位置开始删删多少，在这个位置加入多少元素

替换

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students.splice(1,1,{id:4,score:90});
//原数组
[
    {id:1,score:80},
    {id:4,score:90},
    {id:3,score:70}
]

删除

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students.splice(1,1);
//原数组
[
    {id:1,score:80},
    {id:3,score:70}
]

添加

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
students.splice(1,0,{id:4,score:90});
//原数组
[
    {id:1,score:80},
    {id:4,score:90},
    {id:2,score:50},
    {id:3,score:70}
]

arr.slice(begin[,end]) 复制拷贝数组中的元素，结束位置不取到

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
var newStudents = students.slice(0,2);
[
    {id:1,score:80},
    {id:2,score:50}
]

arr.concat(value1,...,valueN) 做连接可以连多个值

var students1 = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
var students2 = [
    {id:4,score:90},
    {id:5,score:60}
];
var students3 = [
    {id:6,score:40},
    {id:7,score:30}
];
var newStudents = students.concat(students2,students3);
[
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70},
    {id:4,score:90},
    {id:5,score:60},
    {id:6,score:40},
    {id:7,score:30}
]

arr.join([separator])

var emails = ["[email protected]","[email protected]","[email protected]"];
emails.join(";");//"[email protected];[email protected];[email protected]"

问题：改成绩单，但保留原来的成绩单

var scores = [60,70,80,90];
var newScores = [];
var addScore = function(item,index,array){
    newScores.push(item+5);
};
scores.forEach(addScore);
newScores;//[65,75,85,95]

arr.map(callback[,thisArg]) 将返回都加到一个新数组中去

var scores = [60,70,80,90]
var addScore = function(item,index,array){
    return item+5;
};
scores.map(addScore);//[65,75,85,95]

arr.reduce(callback,[initialValue])接收一个函数作为累加器（accumulator），数组中的每个值（从左到右）开始缩减，最终为一个值。

var students = [
    {id:1,score:80},
    {id:2,score:50},
    {id:3,score:70}
];
var sum = function(previousResult,item,index,array){
    return previousResult + item.score;
};
students.reduce(sum,0);//200

讨论：对一个数组（每项都是数值）求和，有哪些方法？