变量与数据类型

声明变量var

var 变量名 = 值;

<script>
        var s = "hello world";
        var s1 = "hello",s2 = "world";
        console.log(s);
        console.log(s1);
        console.log(s2);
    </script>

变量命名规范

数据类型

数组：[1,2,3]

/*数字*/
var num1=7;
var num2=3.14;

/*字符串*/
var str1='a';/*单双引号都可以，都是字符串*/
var str2="ben";
var str='he says:"he is a superman"';

//布尔值
var isEmpty=false;

//对象
//用键值对key:value描述，有属性和方法
var Student={
            "sno":"101",
            "name":"ljs",
            "age":20,
            sayName:function(){//方法
                console.log("我是"+ this.name)
            },
            /*ES6后对象的方法能简写
            sayName:(){
                console.log("我是"+ this.name)
            }*/
            eat:function(food){
                console.log("吃"+food);
            }
        };
        Student.sayName();//调用方法 我是ljs
        Student.eat("鱼");//吃鱼

//数组
var array1=[98,97,96,60,77,85];
var array2=["ben","ljs","mike"]
var array3=[//存放对象
    {"sno":"101","name":"ljs","age":20},
    {"sno":"102","name":"ben","age":22}
];
var array4=new Array("a","b","c");

<script>
        var arr = ["a","b","c","d","e"];

        //for in遍历数组，while和for遍历就不赘述了
        for(var i in arr){//i是数组arr的下标，0~4
            console.log(arr[i]);
        }

        //for of遍历数组，i是数组的元素，arr[0]~arr[4]
        for(var i of arr){
            console.log(i);
        }
        //map方法遍历数组
        arr.map(function(value,index){//index是索引
            console.log("第"+(index+1)+"个元素是"+value);//value是值，这里是数组元素
        });
    </script>

        var arr = ["a","b","c","d","e"];
        //push方法，添加元素
        arr.push("f");
        console.log(arr);//["a","b","c","d","e","f"]

        var arr = [1,3,6,5,2];
        //sort方法排序
        var arrnew = arr.sort();
        console.log(arrnew);//[1,2,3,5,6]

        var arr = [1,3,6,5,2];
        //filter方法，过滤出符合条件的数组元素并放到另一个数组中
        var newarr = arr.filter(function(item){
            if(item>=3){
                return item;//放到新的数组中
            }
        });
        console.log(newarr);//[3,6,5]

        var arr = ["a","b","c"];
        //join方法，转换成字符串
        var str1 = arr.join();
        var str2 = arr.join("");//用""连接元素
        console.log(str1);//a,b,c
        console.log(str2)//abc

        var str="2022-10-23";
        var arr1 = str.split("-");//以"-"为分界将字符串分成数组元素
        var arr2 = str.split("");//每个字符都是一个数组元素
        var arr3 = str.split();//整个字符串是一个数组元素
        console.log(arr1);//['2022', '10', '23']
        console.log(arr2);//['2', '0', '2', '2', '-', '1', '0', '-', '2', '3']
        console.log(arr3);//['2022-10-23']

//null
var student = null;//用来占位，确保语法正确，目前没有意义

//undefined
未赋值的变量的值
var v;//v的值为undefined

//调用变量
//基本数据类型
console.log(num1);//console是控制台，log是打印的意思
console.log(str1);
console.log(isEmpty);

//数组用下标显示
console.log(array1[0]);

//对象用.
console.log(Student.sno);
console.log(Student.name);
console.log(Student.age);

//嵌套数据
console.log(array3[0].sno,array3[0].name,array3[0].age);

关于运算的补充

<script>
        var num=10
        console.log(num++ + ++num)//11+11

        console.log(1 == "1")//true == 比较数值而不比较数据类型
        console.log(1 != "1")//false
        console.log(1 === "1")//false ===恒等于 比较数值和数据类型
        console.log(1 !== "1")//true
    </script>