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let ,const功能与应用场景对象解析与数组解构
零龙
Original
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let与const声明的变量的特点

  • let

  • 1,let定义的变量不会预解析,必须先声明再使用,否则会报错
  • 2,let不能定义已经定义过的变量(无论之前是用var定义的还是let或者const定义的)
  • 3,let是块级作用域,函数内部使用let定义后,对函数外部无影响,简单说就是在一个{}里面生效
  • 4,由于let是块级作用域,在循环绑定事件过程中let会在这个循环中生效,再次循环时let会重新定义生效
  • const

  • 1,const定义的变量不会预解析,必须先声明再使用,否则会报错
  • 2,const定义的变量不允许修改,但是,在数组里面,const的值是允许被修改的,这是因为const存储的是地址,值的内容可以变化

实例:

  1. <!DOCTYPE html>
  2. <html lang="en">
  3. <head>
  4. <meta charset="UTF-8">
  5. <meta name="viewport" content="width=device-width, initial-scale=1.0">
  6. <meta http-equiv="X-UA-Compatible" content="ie=edge">
  7. <title>Document</title>
  8. </head>
  9. <body>
  10. <script>
  11. //在学习的script中使用var定义变量
  12. console.log(a);
  13. var a = 12 ;
  14. //let用同样的方式就会报错
  15. // console.log(b);
  16. let b = 20;
  17. //报错:Uncaught ReferenceError: Cannot access 'b' before initialization
  18. //-----------------------------
  19. let c = 25;
  20. //let声明过的变量不能被重新赋值
  21. //报错:Uncaught SyntaxError: Identifier 'c' has already been declared
  22. //-----------------------------
  23. function num()
  24. {
  25. let d = 15 ;
  26. return (d * 3) + c;
  27. }
  28. console.log(num());
  29. //let作用域:在函数内可以调用函数外的let,反之外不能调用内
  30. //-----------------------------
  31. var ul = document.createElement("ul");
  32. ul.setAttribute("id","nav");
  33. document.body.appendChild(ul);
  34. for(let i = 1;i < 5; i++)
  35. {
  36. var lis =document.createElement("li");
  37. ul.appendChild(lis);
  38. console.log(i); // 正常输出1,2,3,4
  39. }
  40. // console.log(i); //报错:i is not defined
  41. //let 在使用循环定义变量很实用。
  42. //--------------------------------------
  43. //const声明不可以改变的变量、不能被修改
  44. const e = 100;
  45. console.log(e);
  46. // e = e-10; //报错:Assignment to constant variable.
  47. const arr = ['1','2','3','4','5'];
  48. arr[2]='我是数字';
  49. console.log(arr);
  50. //const 定义的数字是可以被修改的
  51. //暂时性死区:
  52. var v =12;
  53. if(true)
  54. {
  55. //console.log(v); //报错:Cannot access 'v' before
  56. const v = 24;
  57. console.log(v); //重新定义v,输出 24
  58. }
  59. console.log(v); //输出 12
  60. </script>
  61. </body>
  62. </html>

解析

  • 使用let对数组及对象解构,可以方便的设置和更新数组。

实例

  1. <!DOCTYPE html>
  2. <html lang="en">
  3. <head>
  4. <meta charset="UTF-8">
  5. <meta name="viewport" content="width=device-width, initial-scale=1.0">
  6. <meta http-equiv="X-UA-Compatible" content="ie=edge">
  7. <title>Document</title>
  8. </head>
  9. <body>
  10. <script>
  11. //----------------数组解构----------------------------
  12. // let array = ['1','曹操','男','39','cc@qq.com','13999999999'];
  13. // let [id,user,sex,age,email,mobile] = array;
  14. // console.log(id);
  15. // console.log(user);
  16. // console.log(sex);
  17. // console.log(age);
  18. // console.log(email);
  19. //let 可以对数组属性赋值到变量中,数组按索引排列,自定义变量名
  20. //----------------数组解构1-1-----------------------------
  21. // let [username,[age1,sex],email] = ['曹操',[23,'男'],'cc@qq.com'];
  22. // console.log(username);
  23. //----------------数组解构-2-----------------------------
  24. // let array = ['1','曹操','男','39','cc@qq.com','13999999999'];
  25. // let [id,user,sex,age,email,mobile] = [array[0],array[1]='刘备',array[2],array[3],array[4]='lb@qq.com',array[5]='13888888888'];
  26. // console.log(user);
  27. // console.log(email);
  28. // console.log(mobile);
  29. //----------------数组解构-3------------------------------
  30. // let array = ['刘备','男','lb@qq.com'];
  31. // let [user,sex,mobile] = array;
  32. // console.log(array);
  33. //----------------对象解构------------------------------
  34. // let obj = {'user':'曹操','email':'cc@qq.com','mobile':'13999999999'};
  35. // let {user,email,mobile} = obj;
  36. // console.log(user);
  37. // console.log(email);
  38. // console.log(mobile);
  39. //----------------对象解构-1------------------------------
  40. // let user = {
  41. // userName :'刘备',
  42. // email:"lb@qq.com",
  43. // other:{
  44. // age:25,
  45. // sex:'男',
  46. // }
  47. // };
  48. // let {userName,email,other:{age,sex}} = user;
  49. // console.log(userName);
  50. // console.log(email);
  51. // console.log(age);
  52. //区分大小写,
  53. //--------------对象解构-2------------------------------
  54. // let user = '曹操';
  55. // let email = 'cc@qq.com';
  56. // ({user,email} = {user:'刘备',email:'lb@qq.com'});
  57. // console.log(user);
  58. // console.log(email);
  59. //--------------对象解构-3----------------------------------
  60. // let user = {
  61. // userName :'刘备',
  62. // email:"lb@qq.com",
  63. // other:{
  64. // age:undefined,
  65. // sex:'男',
  66. // }
  67. // };
  68. // let {userName,email,other:{age= 30,sex}} = user;
  69. // console.log(age);
  70. //--------------对象解构-4----------------------------------
  71. let user = {
  72. userName :'刘备',
  73. email:"lb@qq.com",
  74. other:{
  75. age:undefined,
  76. sex:'男',
  77. }
  78. };
  79. let {userName:User,email,other:{age=25,sex}}=user;
  80. console.log(User);
  81. console.log(sex);
  82. </script>
  83. </body>
  84. </html>

示例图

Correcting teacher:天蓬老师天蓬老师

Correction status:qualified

Teacher's comments:坚持
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