字符串和数组的常用方法与分支循环作业

字符串的常用方法

let str = "957461674@qq.com";
      // concat():字符串拼接
      let str1 = "我的邮箱：".concat(str);
      console.log(str1); // 我的邮箱：957461674@qq.com
      // slice(start,end):截取字符串,从start开始截取到end，不包含end，第二个参数如果不写则默认截取到最后一个
      let str2 = str.slice(2, 5);
      console.log(str2); // 746
      // substr(start,end) 第二个参数指定获取字符的数量
      let str3 = str.substr(2, 6);
      console.log(str3); // 46167
      // trim() 删除两边的空白字符
      let str4 = "     哈哈哈哈哈    ";
      console.log(str4.trim()); // 哈哈哈哈哈
      // 字符串转数组
      let res = "html JavaScript php vue";
      let res1 = res.split(" "); //  ["html", "JavaScript", "php", "vue"]
      console.log(res1);

数组的常用方法

et arr = [];
      // push：在尾部添加元素
      arr.push(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12);
      console.log(arr); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
      // pop：在尾部删除元素
      arr.pop();
      console.log(arr); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]
      // unshift：在头部增加元素
      arr.unshift(0);
      console.log(arr); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
      // shift：在头部删除元素
      arr.shift();
      console.log(arr); // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
      // join：数组转字符串
      let a = arr.join("</li><li>");
      a = `<ul><li>${a}</li>></ul>`;
      console.log(a);
      document.write(a);
      // 拼接数组：concat()
      console.log(arr.concat(["php"])); //  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, "php"]
      //  splice():取子元素
      console.log(arr.splice(0, 4)); // [1, 2, 3, 4]
      // splice数显数组的增删改操作
      // 1、新增
      console.log(arr);
      let q = arr.splice(2, 0, "哈哈");
      console.log(arr); // [5, 6, "哈哈", 7, 8, 9, 10, 11]
      // 2、新增替换
      let w = arr.splice(2, 2, "嘿嘿", "吼吼");
      console.log(arr); // [5, 6, "哈哈", 7, 8, 9, 10, 11]
      // forEach()
      arr.forEach((item, i) => {
        console.log(item + "---" + i);
      });
      // filter()
      console.log(arr.filter((item) => item % 2 == 0)); // arr.filter((item)=> item % 2 == 0);
      s = [1, 23, 4, 5, 6];
      s1 = s.reduce((prev, next) => {
        return prev + next;
      });
      console.log(s1); // 39

循环

 //  while()
      let color = ["red", "blue", "green"];
      let i = 0;
      while (i < color.length) {
        console.log(color[i]);
        i++;
      }
      console.log("********************************");
      // do while()
      i = 0;
      do {
        console.log(color[i]);
        i++;
      } while (i < color.length);
      // for in 循环对象
      let obj = {
        name: "yajie",
        age: "18",
        emal: "vueadmin@qq.com",
      };
      for (let j in obj) {
        console.log("k = %o,v = %o", j, obj[j]);
      }
      /*
      k = "name",v = "yajie"
       k = "age",v = "18"
       k = "emal",v = "vueadmin@qq.com"
       */
    //   for of
    let w = [1,5,6,98,9];
    for(let e of w){
        console.log(e);
    }

分支结构

let score = 80;
      if (score >= 90 && score <= 100) {
        alert("兄弟，牛批啊考的不错！");
      } else if (score >= 80 && score < 90) {
        alert("good!good!还不错！");
      } else if (score >= 60 && score < 80) {
        alert("啧啧，要加把劲咯！");
      } else if (score > 100 && score < 0) {
        alert("请输入0-100之间的数值");
      } else {
        alert("儒了子，考的什么玩意！");
      }
      switch (true) {
        case score >= 90 && score <= 100:
          alert("兄弟，牛批啊考的不错！");
          break;
        case score >= 80 && score < 90:
          alert("good!good!还不错！");
          break;
        case score >= 60 && score < 80:
          alert("啧啧，要加把劲咯！！");
          break;
        case score > 100 && score < 0:
          alert("请输入0-100之间的数值");
          break;
        default:
          alert("儒了子，考的什么玩意！");
          break;
      }