From getting started to becoming proficient in front-end JavaScript, basic knowledge and practical experience such as advanced ES6 syntax, API, and advanced js in core javascript - JS Basics (4)

Enterprise 2023-10-02 21:36:36 views: null

Let’s start and be the master of time!

Give time to sleep, books, and exercise.

Distributed to flowers, birds, trees, mountains, rivers, lakes and seas,

Share your love for this world,

Instead of wasting yourself on boring people and things.

mind Mapping

function

Why functions are needed

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    span {
      display: inline-block;
      width: 100px;
      padding: 5px 10px;
      border: 1px solid pink;
      margin: 2px;
      border-radius: 5px;
      box-shadow: 2px 2px 2px rgba(255, 192, 203, .4);
      background-color: rgba(255, 192, 203, .1);
      text-align: center;
      color: hotpink;
    }
  </style>
</head>

<body>

  <script>
    // // 1. 外层循环控制行数
    // for (let i = 1; i <= 9; i++) {
    //   // 2. 里层循环控制列数
    //   for (let j = 1; j <= i; j++) {
    //     document.write(`<span>${j} X ${i} = ${i * j}</span>`)
    //   }
    //   // 换行
    //   document.write('<br>')
    // }
    // // 1. 外层循环控制行数
    // for (let i = 1; i <= 9; i++) {
    //   // 2. 里层循环控制列数
    //   for (let j = 1; j <= i; j++) {
    //     document.write(`<span>${j} X ${i} = ${i * j}</span>`)
    //   }
    //   // 换行
    //   document.write('<br>')
    // }
    // // 1. 外层循环控制行数
    // for (let i = 1; i <= 9; i++) {
    //   // 2. 里层循环控制列数
    //   for (let j = 1; j <= i; j++) {
    //     document.write(`<span>${j} X ${i} = ${i * j}</span>`)
    //   }
    //   // 换行
    //   document.write('<br>')
    // }
    // 声明
    function sheet99() {
      for (let i = 1; i <= 9; i++) {
        // 2. 里层循环控制列数
        for (let j = 1; j <= i; j++) {
          document.write(`<span>${j} X ${i} = ${i * j}</span>`)
        }
        // 换行
        document.write('<br>')
      }
    }
    // 调用
    sheet99()
    sheet99()
    sheet99()
    sheet99()
  </script>
</body>

</html>

Code reuse to generate multiple 99 multiplication tables

Function usage

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    // let num = 10
    // console.log(num)
    // 1. 函数的声明   
    function sayHi() {
      console.log('hi~~~')
    }
    // 2. 函数调用   函数不调用，自己不执行
    sayHi()
    sayHi()
    sayHi()
  </script>
</body>

</html>

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>

  <script>
    // 1. 求2个数的和
    // function getSum() {
    //   let num1 = +prompt('请输入第一个数')
    //   let num2 = +prompt('请输入第二个数')
    //   console.log(num1 + num2)
    // }
    // getSum()
    // 2. 求 1~100 累加和
    function getSum() {
      let sum = 0
      for (let i = 1; i <= 100; i++) {
        sum += i
      }
      console.log(sum)
    }
    getSum()
  </script>
</body>

</html>

Function parameter passing

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>

  <script>

    // 2. 求 1~100 累加和
    // function getSum(end) {   // end = 50
    //   // console.log(end)
    //   let sum = 0
    //   for (let i = 1; i <= end; i++) {
    //     sum += i
    //   }
    //   console.log(sum)
    // }
    // getSum(50)  // 1~50
    // getSum(100)  // 1~100

    function getSum(start, end) {   // end = 50
      // 形参  形式上的参数  
      // console.log(end)
      let sum = 0
      for (let i = start; i <= end; i++) {
        sum += i
      }
      console.log(sum)
    }
    getSum(1, 50)  // 调用的小括号里面 实参 - 实际的参数
    getSum(100, 200)  // 实参 - 实际的参数
  </script>
</body>

</html>

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    // 1. 封装函数
    // 给一个参数的默认值
    function getArrSum(arr = []) {
      // console.log(arr)
      let sum = 0
      for (let i = 0; i < arr.length; i++) {
        sum += arr[i]
      }
      console.log(sum)
    }
    getArrSum([1, 2, 3, 4, 5])
    getArrSum([11, 22, 33])
    getArrSum()  // 0
  </script>
</body>

</html>

function return value

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    // // 函数的返回值
    // function fn() {
    //   return 20
    // }
    // // fn() 调用者 相当于执行了    fn()   = 20
    // // return 的值返回给调用者
    // // console.log(fn())
    // // let num = prompt('请输入数字')
    // let re = fn()
    // console.log(re)

    // 求和函数的写法
    function getTotalPrice(x, y) {
      return x + y
      // return 后面的代码不会被执行
    }
    // console.log(getTotalPrice(1, 2))
    // console.log(getTotalPrice(1, 2))
    let sum = getTotalPrice(1, 2)
    console.log(sum)
    console.log(sum)

    function fn() {

    }
    let re = fn()
    console.log(re)  // undefined
  </script>
</body>

</html>

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

</head>

<body>
  <div></div>
  <script>
    // 1. 求最大值函数
    // function getMax(x, y) {
    //   return x > y ? x : y
    // }
    // let max = getMax(11, 234)
    // console.log(max)

    // //  2. 求任意数组的最大值，并且返回
    // function getArrValue(arr = []) {
    //   // (1)先准备一个max变量存放数组的第一个值
    //   let max = arr[0]
    //   // (2) 遍历比较
    //   for (let i = 1; i < arr.length; i++) {
    //     if (max < arr[i]) {
    //       max = arr[i]
    //     }
    //   }
    //   // (3) 返回值
    //   return max
    // }

    // // let max = getArrValue([1, 3, 5, 7, 9])
    // // let num = prompt('请输入')
    // let max = getArrValue([11, 3, 55, 7, 29])
    // console.log(max)
    //  3. 求任意数组的最大值和最小值，并且返回
    function getArrValue(arr = []) {
      // (1)先准备一个max变量存放数组的第一个值
      let max = arr[0]
      let min = arr[0]  // 最小值
      // (2) 遍历比较
      for (let i = 1; i < arr.length; i++) {
        // 最大值
        if (max < arr[i]) {
          max = arr[i]
        }
        // 最小值
        if (min > arr[i]) {
          min = arr[i]
        }
      }
      // (3) 返回值  返回的是数组
      return [max, min]
      // return min
    }
    let newArr = getArrValue([11, 3, 55, 7, 29])
    console.log(`数组的最大值是: ${newArr[0]}`)
    console.log(`数组的最小值是: ${newArr[1]}`)
  </script>
</body>

</html>

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    // function getSum(x, y) {
    //   return x + y
    //   // 返回值返回给了谁？ 函数的调用者  getSum(1, 2)
    //   // getSum(1, 2) = 3
    // }
    // // let result = getSum(1, 2) = 3
    // // let num = parseInt('12px')
    // let result = getSum(1, 2)
    // console.log(result)
    // 1. 函数名相同， 后面覆盖前面

    // function fn() {
    //   console.log(1)
    // }
    // function fn() {
    //   console.log(2)
    // }
    // fn()
    // 2. 参数不匹配

    function fn(a, b) {
      console.log(a + b)
    }
    // (1). 实参多余形参   剩余的实参不参与运算
    // fn(1, 2, 3)
    // (2). 实参少于形参   剩余的实参不参与运算
    fn(1)   // 1 + undefined  = NaN 
  </script>
</body>

</html>

Scope

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    let num = 10  // 1. 全局变量
    console.log(num)
    function fn() {
      console.log(num)
    }
    fn()

    // 2. 局部变量
    function fun() {
      let str = 'pink'
    }
    console.log(str)  // 错误
  </script>
</body>

</html>

Within a function, the first local variable that appears is treated as a global variable.

There is no let variable declaration keyword, or even overwriting global variables with the same name (strongly not recommended)

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    // let num = 20
    // function fn() {
    //   num = 10  // 全局变量来看  强烈不允许
    // }
    // fn()
    // console.log(num)

    // function fun(x, y) {
    //   // 形参可以看做是函数的局部变量
    //   console.log(x)
    // }
    // fun(1, 2)
    // console.log(x)  // 错误的

    // let num = 10
    function fn() {
      // let num = 20
      function fun() {
        // let num = 30
        console.log(num)
      }
      fun()
    }
    fn()
  </script>
</body>

</html>

anonymous function

Third-party js files (named functions that are executed immediately) are worried about variable name conflicts:

<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>

<body>
  <script>
    // age = age + 1
    // 1. 用户输入
    let second = +prompt('请输入秒数:')
    // 2.封装函数
    function getTime(t) {
      // console.log(t)  // 总的秒数
      // 3. 转换
      // 小时：  h = parseInt(总秒数 / 60 / 60 % 24)
      // 分钟：  m = parseInt(总秒数 / 60 % 60)
      // 秒数: s = parseInt(总秒数 % 60) 
      let h = parseInt(t / 60 / 60 % 24)
      let m = parseInt(t / 60 % 60)
      let s = parseInt(t % 60)
      h = h < 10 ? '0' + h : h
      m = m < 10 ? '0' + m : m
      s = s < 10 ? '0' + s : s
      // console.log(h, m, s)
      return `转换完毕之后是${h}小时${m}分${s}秒`
    }
    let str = getTime(second)
    document.write(str)
    console.log(h)
  </script>
</body>

</html>

notes

Go to the knowledge condensed notes:

JavaScript Basics-Day 4 Notes-CSDN Blog

Guess you like

Origin blog.csdn.net/upgrade_bro/article/details/133187197
Recommended
TIOBE May list: Fortran “resurrected” into Top 10
GCC 14.1 released
Ranking
B. Little Girl and Game【1300 / 回文字符串 博弈论】
CIKERS Shane 20190613
"Javascript advanced programming" study notes - the constructor and prototype
beeline hiveserver2 start
springboot - Automatically backup mysql data every day
Data Storage Full Solution--Detailed Persistence Technology
Detailed Explanation of Spring Web MVC DispatcherServlet—Official Original
TCP / IP protocol layers structure and function
Command type literal pos: unknown； Fallback type literal pos: unknown] with root cause
Design of multifunctional curtain controller with indoor anti-theft alarm
Daily
More
2024-05-08(18)
2024-05-07(34)
2024-05-06(6)
2024-05-05(0)
2024-05-04(18)
2024-05-03(8)
2024-05-02(0)
2024-05-01(4)
2024-04-30(36)
2024-04-29(5)

Code World

© 2018 codetd.com