2022.7.30 js notes Operators and flow controllers, loops

一、自增、自减,比较,逻辑

        1.自增、自减

        前置操作（自减同理）：The expression is executed first

    var a = 1;
    b = 1 + ++a;
    console.log(b); // 3

        后置操作（自减同理）：Executed at the end of the expression

    var a = 1;
    var b = 1 + a++;
    console.log(b); // 2

        2.比较运算符

        （类型等于）== ：强制类型转换,will coerce the values ​​on both sides to  数字或布尔值 类型再进行比较.

       （全等于） === ：Non-coercive type conversion,Compare the values ​​on both sides without converting them.

        3.逻辑运算符

        逻辑与&& ：当左边为 false Returns the current value immediately,当左边为 true Returns the right-hand value immediately.

        逻辑或|| ：When there is one on both sides true 立即返回 true ,反之返回 false .

        逻辑非! ：返回相反值.

        优先级：&&优先级最高,So prioritize,The way to increase the priority is to add （）.

二、if、if/else、三元表达式

        1.if 用法

    var person = "boy";
    if (person === "girl") {
      console.log("hi,girl");
    }
    console.log("hi,boy");

        如果只有一条代码块,可以不用写{}.

        2.if/else 用法

    var person = "boy";
    if (person === "girl") {
      console.log("hi,girl");
    } else {
      console.log("hi,boy");
    }

        3.三元表达式（非真即假）

    var person = "boy";
    person === "girl" ? console.log("girl") : console.log("boy");

三、swith 用法

    var weather = "下雪";
    switch (weather) {
      case "晴朗": //case 后的值和 swith() The value contrast is all equal
        console.log("去打篮球");
        break;
      case "下雨":
        console.log("收衣服");
        break;
      case "下雪":
        console.log("堆雪人");
        break;
      default:
        console.log("学习");
        break; //加不加都行
    }

        有3,4when the above conditions are met,You can consider using it⽤ switch .

四、for 循环

        for(语句1;语句2;语句3){

        The block of code to repeat

}

        // 语句1声明一个变量

        // 语句2Specifies the loop breakout condition

        // 语句3Control variable changes

流程：

        语句1在循环（代码块）开始前执行

        语句2Defines the end loop（代码块）的一个条件

        语句3会在循环（代码块）运行完成后执行

遍历数组：

    var sum = ["a", "b", "c", "d", "e"];
    for (let s = 0; s < sum.length; s++) {
      console.log(sum[s]);
    }
// a b c d e

计算：

    var sum = 0;
    for (let a = 0; a < 10; a++) {
      sum += a;
    }
    console.log(sum);

五、while 循环（不常用）

1. while 用法：

    var a = 1;
    var sum = 0;
    while (a < 10) {
      sum += a;
      a++;
    }
    console.log(sum);

2. do while 用法（Guaranteed to cycle at least once）

    var a = 1;
    var sum = 0;
    do {
      sum += a;
      a++;
    } while (a < 10);
    console.log(sum);

六、练习

        1.打印1-100之间7的倍数的个数及总和

    var sum = 0;
    var t = 0;
    for (let a = 1; a <= 100; a++) {
      if (a % 7 === 0) {
        t++;
        sum += a;
      }
    }
    console.log("个数：", t, "总和", sum);

        2.使⽤for循环输出⼀下图形

*

* *

* * *

* * * *

* * * * * 

    for (let a = 0; a < 5; a++) {
      for (let b = 0; b < a + 1; b++) {
        document.write("* ");
      }
      document.write("</br>");
    }