【前端】语法学习

<script>
    // var 定义变量
    {
        var a = 10
        var b = "zhangsan"
    }
    console.log(a , b)
    var a = 20
    var b = 30
    console.log(a , b)
    a = 40
    b = 50
    console.log(a , b)

    // let 定义变量
    let c = 60
    let d = "lisi"
    // let c = 70 -- let 定义的变量不能被二次定义，但是可以二次赋值
    c = 70
    d = 80
    console.log(c , d)

    {
        c = 80
        d = 'wangwu'
        console.log("内部：" + c , d)

        let e = 1
        let f = 2
    }
    console.log("外部：" + c , d)
    // console.log(e , f) -- 不能被访问


    // 对象
    var user = {name:"zhangsan",age:20}
    console.log(user)

    // 常量 const
    const name = "wangwu"
    const age = 23

    // 常规语法
    user = {name:name,age:age}
    console.log(user.name + "---" + user.age)

    // es6语法
    user = {name,age}
    console.log(user.name + "***" + user.age)
   
    var dog = {name:"xiaobai",age:3,sex:1}
    var cat = {name:"xiaohei",age:2,email:"hhhh@qq.com"}

    // 合并对象、修改属性
    var merge = {...dog}
    console.log(merge)
    var merge = {...cat}
    console.log(merge)
    var merge = {...dog,...cat}
    console.log(merge)
    var merge = {...cat,...dog}
    console.log(merge)
    var merge = {...dog,name:"xiaohuang"}
    console.log(merge)

    // 方法
    var person = {
        name:"zhangsan",
        age:20,
        eat(name){
            alert(name + "吃饭!")
        },
        sleep(){
            alert(this.name + "睡觉觉!")
        }
    }

    person.eat("zhaoliu")
    person.sleep()

    // 箭头函数: 参数 => 函数体
    var f1 = num => num
    alert(f1("hello world"))

    var f2 = (a,b) => a + b
    alert(f2(1,2))
    alert(f2("hello","world"))

</script>