javascript数据类型和分支循环


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    <title>javascript数据类型和分支循环</title>
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  <body>
    <script>
      //   javascript数据类型分为
      //   1、原始类型： Number String Boolean undefined null
      //   2、引用类型：array object function
      // number
      console.log("这是number:" + 123);
      //string
      console.log("这是string:" + "123");
      //boolean
      console.log("这是boolean:" + false);
      //undefined
      let a;
      console.log("这是undefined:" + a);
      //null
      let b = null;
      console.log("这是null:" + b);
      //array
      let arr = ["小明", "男", "16", "16cm"];
      console.log("这是array:" + arr);
      let obj1 = [
        { name: "手机", num: 2, price: 5000 },
        { name: "电脑", num: 5, price: 6000 },
        { name: "相机", num: 4, price: 2000 },
      ];
      console.log("这是array:" + obj1[0].name);
      //object
      let obj = {
        name: "Tom",
        age: 18,
        height: "180cm",
        fun: function () {
          console.log("这是一个function.");
        },
      };
      console.log("这是一个object:" + obj["age"]);
      console.log("这是一个object:" + obj.age);
      obj.fun();
      //查看数据类型用typeof
      console.log(typeof function () {});
      console.log(arr instanceof Object);
      //回调函数
      function f1() {}
      function f2(f1) {
        console.log("这是回调函数" + typeof f1);
      }
      f2(f1);
      //这是闭包函数
      function f3() {
        return function () {
          return "hello world";
        };
      }
      console.log("这是闭包函数" + f3()());
    </script>
    <script>
      //分支和循环
      // 1、单分支
      if (true) {
        console.log("这是一个单分支");
      }
      //2、双分支
      if (false) {
        console.log("这个是双分支,真的");
      } else {
        console.log("这个是双分支,假的");
      }
      //3、三元运算
      let res;
      isNaN(123) ? (res = "是") : (res = "不是");
      console.log("这" + res + "三元运算");
      //多分支
      let age = 20;
      if (age < 18) {
        console.log("年龄小于18岁");
      } else if (age >= 18 && age < 25) {
        console.log("正是青春年华");
      } else if (age >= 25) {
        console.log("差不多了");
      }
      //switch判断分支
      //必须要用true做为switch入口判断条件
      switch (true) {
        case age < 18:
          console.log("年龄小于18岁");
          breack;
        case age >= 18 && age < 25: {
          console.log("正是青春年华");
          break;
        }
        default: {
          console.log("差不多了");
        }
      }
      //for循环，for of 循环 ，for in 循环
      arr = [9, 4, 2, 7, 4];
      obj = {
        product: "电视",
        num: 3,
        price: 300,
        fun: function () {},
      };
      //for循环
      for (let i = 0; i < arr.length; i++) {
        console.log("for循环:" + arr[i]);
      }
      //for of 循环
      for (let item of arr) {
        console.log("for of循环:" + item);
      }
      for (let item of arr.values()) {
        console.log("for of循环值:" + item);
      }
      for (let item of arr.keys()) {
        console.log("for of循环键:" + item);
      }
      //for in 循环
      for (let key in obj) {
        console.log("for in循环键:" + key);
      }
      //这里只能发obj[key]访问，不能用obj.key .obj[key],key不能有引号
      for (let key in obj) {
        console.log("for in循环值:" + obj[key]);
      }
    </script>
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