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let、const与解构赋值
王佳祥
Original
1096 people have browsed it

let、const与解构赋值

一、let

  • es6新增了let命令,用来声明变量,它的用法类似var,但是所声明的变量只在let命令所在的代码块内有效。
  1. {
  2. let a = 'let变量';
  3. var b = 'var变量';
  4. }
  5. console.log(a);//a is not defined
  6. console.log(b);//正确输出
  1. 上面代码块,分别用letvar声明了两个变量,在代码块之外调用这两个变量时,let声明的变量就会报错,而var声明的变量正常输出,这说明了let只在它所在的代码块内有效。
  • let禁止重复声明
  1. //let声明变量
  2. let num;
  3. //禁止重复声明
  4. let num;//会报错
  • 不存在变量声明提升
  1. //不存在变量声明提升
  2. console.log(username);//会报错
  3. let username = "wang";
  4. function fn() {
  5. console.log(username);
  6. let username = "jack";
  7. }
  8. console.log(fn());//会报错
  • 支持块作用域
  1. //支持块作用域
  2. if (true) {
  3. var price = 99;
  4. //let price = 50;
  5. }
  6. console.log(price);
  7. let lis = document.querySelectorAll("li");
  8. for (let i = 0; i < lis.length; i++) {
  9. lis[i].addEventListener("click", function () {
  10. console.log("点击了第" + i + "个");
  11. });
  12. }


二、const

  • const声明只读变量:常量
  1. //const声明只读变量:常量
  2. //1.const常量,在一个脚本的声明周期内,禁止更新,所以在声明时必须初始化
  3. const NATION = "中国";
  4. //2.不允许重复声明
  5. //const NATION ="CHINA";
  6. //3.支持块作用域
  7. if (true) {
  8. const EMAIL = "wukong@qq.com";
  9. console.log(EMAIL);
  10. }
  11. //console.log(EMAIL);会报错
  12. /*
  13. let 与 const相同之处
  14. 1.禁止重复声明
  15. 2.支持块作用域
  16. 不同之处,let允许更新,而const禁止更新
  17. 一种风格就是全部都用const,除非这个数据后期会有改变
  18. 可能会更新的或常用的都使用let,除非不会或不太可能被修改的采用const
  19. 有一些数据类型强烈推荐使用const
  20. 对象或数组
  21. */
  22. const arr = [1, 2, 3];
  23. console.log(arr);
  24. arr[1] = 20;
  25. console.log(arr);
  26. //arr = ["a","b"]这样更新会报错
  27. const obj = { x: "red", y: "green" };
  28. console.log(obj);
  29. obj.x = "blue";
  30. console.log(obj);
  31. //obj = {};//这样更新会报错,不能直接赋值
  32. const body = document.body;
  33. body.style.backgroundColor = "yellow";

三、var/let/const在全局作用域绑定上的区别

  1. //var 全局
  2. var email = "wukong@qq.com";
  3. console.log(email);
  4. console.log(window.email);
  5. var email = "bajie@qq.com";
  6. console.log(email);
  7. //污染全局环境
  8. //尽量不要创建全局变量,可以创建临时变量
  9. //var role = "custom";
  10. //临时变量就是创建立即执行函数
  11. (function () {
  12. var role = "custom";
  13. })();
  14. //这样就不会显示在全局环境中
  15. //在当前全局环境中创建一个命名空间
  16. //如果在函数对象上添加就是静态成员
  17. var ns = {};
  18. ns.id = 10;
  19. ns.username = "wang";
  20. ns.email = "wang@qq.com";
  21. console.log(window.ns.id, window.ns.username, window.ns.email);
  22. /*------------------------------------------------------*/
  23. //let
  24. let colors = ["red", "green", "blue"];
  25. const BRAND = "HUAWEI";
  26. console.log(colors, BRAND);
  27. //使用let/const声明的变量不会成为全局对象window的属性
  28. console.log(window.colors, window.BRAND); //返回undefined
  29. var name = "hello world";
  30. let name = "hello php";
  31. //let和const声明的变量只会出现在词法作用域/块作用域/静态作用域中
  32. //在全局环境下,var,let,const行为类似

四、解构

  1. //解构主要是针对对象和数组
  2. const obj = { x: 1, y: 2, z: 3 };
  3. /* for (k in obj) {
  4. console.log(k);
  5. } */
  6. //模板解构
  7. //对象
  8. const { x, y, z } = { x: 1, y: 2, z: 3 };
  9. console.log(x, y, z);
  10. //数组
  11. const [a, b, c] = ["a", "b", "c"];
  12. console.log(a, b, c);
  13. //解构的目的:将集合数据按规则打散到一些独立的变量中
  14. //一.传统方式
  15. const product = {
  16. name: "电脑",
  17. price: 5000,
  18. };
  19. let name = product.name;
  20. let price = product.price;
  21. console.log("%s:%c %d", name, "color:red", price);
  22. //二.对象解构
  23. const user = {
  24. username: "wang",
  25. email: "wang@qq.com",
  26. };
  27. //解构语法:解构变量声明 = 要被解构的数据
  28. //let { username, email } 就是结构变量声明
  29. let { username, email } = {
  30. username: "wang",
  31. email: "wang@qq.com",
  32. };
  33. console.log("%s (%s)", username, email);
  34. //1.对象的属性名必须与左边解构变量声明模板中的变量名同名
  35. //2.解构变量必须初始化
  36. let { q, w } = {};
  37. //3.可以将左侧的变量声明看成右侧的赋值模板
  38. //三.解构表达式
  39. let a1 = 10;
  40. let b1 = 20;
  41. console.log(a1, b1);
  42. a1 = 150;
  43. b1 = 300;
  44. console.log(a1, b1);
  45. //换种方式来更新
  46. //1.场景1:用来更新变量
  47. ({ a1, b1 } = { a1: 15, b1: 60 });
  48. console.log(a1, b1);
  49. //2.场景2
  50. function out({ m: x, n: y }) {
  51. console.log(x, y);
  52. x = 20;
  53. y = 40;
  54. console.log(x, y);
  55. }
  56. out({ m: 8, n: 8 });
  57. //四.解构声明中设置默认值
  58. let stu = {
  59. stuName: "wang",
  60. gender: "male",
  61. };
  62. let { stuName, gender = "famale", age = 15 } = stu;
  63. console.log(stuName, gender, age);
  64. //五.解构声明中使用变量别名
  65. const book = {
  66. name: "ES6开发指南",
  67. price: 109,
  68. };
  69. let { name: bookname, price: bookprice } = book;
  70. console.log(bookname, bookprice);

五、嵌套对象解构

  1. //对象的属性值也可以是对象或其它复杂类型、
  2. const stu = {
  3. name: "wang",
  4. course: {
  5. php: {
  6. level: "basis",
  7. grade: 80,
  8. },
  9. front: {
  10. level: "advance",
  11. grade: 70,
  12. },
  13. },
  14. };
  15. console.log(stu);
  16. let {
  17. course: { php },
  18. } = stu;
  19. console.table(php);
  20. let {
  21. course: {
  22. php: { level },
  23. },
  24. } = stu;
  25. console.table(level);
  26. let {
  27. course: {
  28. front: { level: frontlevel },
  29. },
  30. } = stu;
  31. console.table(frontlevel);
  32. //多次解构
  33. let {
  34. course,
  35. course: { php: phpinfo },
  36. course: {
  37. php: { level: levelinfo },
  38. },
  39. } = stu;
  40. console.log(course, php, level);

六、数组解构

  1. //数组解构
  2. const [c1, c2, c3] = ["上海", "北京", "广州"];
  3. console.log(c1, c2, c3);
  4. //1.数组解构声明
  5. let user = [1, "admin", "admin@qq.com"];
  6. let [id, name, email] = user;
  7. console.log(id, name, email);
  8. let [, username, ,] = user;
  9. //数组解构和对象解构必须要初始化
  10. console.log(name);
  11. //2.数组解构表达式:更新数组元素
  12. let lesson = "es6";
  13. let grade = 60;
  14. //不要加let,不要加括号([lesson, grade] = ["php", 90]);
  15. [lesson, grade] = ["php", 90];
  16. console.log(lesson, grade);
  17. //3.使用默认值
  18. let [brand, model, color = "red"] = ["huawei", "p40"];
  19. console.log(brand, model, color);
  20. //4.在函数参数中也可以使用数组解构
  21. function add([x, y = 100]) {
  22. return x + y;
  23. }
  24. console.log(add([50, 100]));
  25. console.log(add([100]));
  26. //5.数组嵌套解构
  27. let [a1, a2, [a3, a4], a5] = [10, 20, [30, 40], 50];
  28. console.log(a1, a2, a3, a4, a5);

七、学习总结

1.let

  • 禁止重复声明

  • 不存在变量提升

  • 支持块作用域

2.const

  • const声明只读变量:常量

  • const声明变量必须初始化

  • 禁止重复声明

  • 支持块作用域

  • 禁止更新

  • 可能会更新的或常用的都使用let,除非不会或不太可能被修改的采用const

3.var/let/const在全局作用域绑定上的区别

  • 使用var声明的变量会出现在全局环境中

  • 可以用立即执行函数来创建临时变量,这样就不会污染全局环境

  1. (function () {
  2. var role = "custom";
  3. })();
  • 也可以创建一个命名空间
  1. var ns = {};
  2. ns.id = 10;
  3. ns.username = "wang";
  4. ns.email = "wang@qq.com";
  5. console.log(window.ns.id, window.ns.username, window.ns.email);
  • 使用let/const声明的变量不会成为全局对象window的属性
  • let和const声明的变量只会出现在词法作用域/块作用域/静态作用域中

4.解构

  • 解构主要是针对对象和数组

  • 解构的目的:将集合数据按规则打散到一些独立的变量中

  • 解构语法:解构变量声明 = 要被解构的数据

  1. let { username, email } = {username: "wang",email: "wang@qq.com",};
  • 对象的属性名必须与左边解构变量声明模板中的变量名同名

  • 解构变量必须初始化

  • 解构表达式可以用来更新变量

  1. let a1 = 10;
  2. let b1 = 20;
  3. ({ a1, b1 } = { a1: 15, b1: 60 });
  4. console.log(a1, b1);
  • 解构声明中可以设置默认值
  1. let stu = {stuName: "wang",gender: "male",};
  2. let { stuName, gender = "famale", age = 15 } = stu;
  3. console.log(stuName, gender, age);
  • 解构声明中可以使用变量别名
  1. const book = {name: "ES6开发指南",price: 109,};
  2. let { name: bookname, price: bookprice } = book;
  3. console.log(bookname, bookprice);

5.嵌套对象解构

  • 对象的属性值也可以是对象或其它复杂类型
  1. const stu = {name: "wang",
  2. course: {
  3. php: {level: "basis",grade: 80,},
  4. front: {level: "advance",grade: 70,},
  5. },
  6. };
  7. let {course: { php },} = stu;
  8. console.table(php);
  9. let { course: { php: { level },},} = stu;
  • 多次解构
  1. let {
  2. course,
  3. course: { php: phpinfo },
  4. course: { php: { level: levelinfo },},
  5. } = stu;
  6. console.log(course, php, level);

6.数组解构

  • 数组解构和对象解构必须要初始化

  • 数组解构表达式:可以更新数组元素

  1. let lesson = "es6";
  2. let grade = 60;
  3. //不要加let,不要加括号([lesson, grade] = ["php", 90]);
  4. [lesson, grade] = ["php", 90];
  5. console.log(lesson, grade);
  • 也可以使用默认值
  1. let [brand, model, color = "red"] = ["huawei", "p40"];
  2. console.log(brand, model, color);
  • 在函数参数中也可以使用数组解构
  1. function add([x, y = 100]) {
  2. return x + y;
  3. }
  4. console.log(add([50, 100]));
  5. console.log(add([100]));
  • 数组嵌套解构
  1. let [a1, a2, [a3, a4], a5] = [10, 20, [30, 40], 50];
  2. console.log(a1, a2, a3, a4, a5);
Correcting teacher:天蓬老师天蓬老师

Correction status:qualified

Teacher's comments:解构的功能非常强大, 应用也很广泛, 多查查资料去学习它
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