变量

var 可以重复定义,不能限制修改，没有块级作用域
let 不能重复定义，变量，有块级作用域
const 不能重复定义，常量不能重新赋值，有块级作用域

建议永远抛弃var

  1. //重复赋值
  2. var a = 1
  3. a = 2
  4. console.log(a) //2
  5. let b = 1
  6. b = 2
  7. console.log(b)// 2
  8. const c = 1
  9. c = 2
  10. console.log(c) // Assignment to constant variable
  12. //重复定义
  13. var a1 = 1
  14. var a1 = 2
  15. console.log(a)
  16. let b1 = 1
  17. let b1 = 2
  18. console.log(b1) //Identifier 'b1' has already been declared
  19. const c1 = 1
  20. const c1 = 2
  21. console.log(c1)//Identifier 'c1' has already been declared
  23. //块级作用域
  24. if(1){
  25.     var a2 = 1
  26.     let b2 = 1
  27.     const c2 = 1
  28. }
  29. console.log(a2)
  30. console.log(b2) // b2 is not defined
  31. console.log(c2) // c2 is not defined
  33. //let没有变量提升
  34. function a(){
  35.     console.log(arr)
  36.     console.log(brr);
  37.     let arr = 1;
  38.     var brr = 1;
  39. }
  40. a();// arr is not defined
  42. //const必须赋值
  43. const a  //Uncaught SyntaxError: Missing initializer in const declaration

解构赋值

左右两边解构必须一样，右边必须是合法东西
左右两边赋值必须同步完成

  1. let [a,b,c] = [12,5,8]
  2. let {a,b,c} = {a:12,b:5,c:8}
  4. let [{a,b}] = [{"a":1,"b":2}]
  5. console.log(a,b) //1,2
  7. let [name,age] = ['niliv',3]
  8. let p = {name,age}
  9. console.log(p)
  10. // {name: "niliv", age: 3}
  12. let [n,{a,b},m] = [12,{a:1,b:2},12]
  13. console.log(a,n,m,b)
  14. let {a,b} = {1,2} //Unexpected token
  16. let [o,p];
  17. [o,p] = [1,2] //Missing initializer in destructuring declaration
  19. let [x,y] = [1,2];  //数组后面必须分号
  20. [x,y] = [y,x]
  22. //默认值
  23. let [c,b=2] = [3]
  24. //c 3
  25. //b 2
  27. let [x,y=2] = [3,4]
  28. //x 3
  29. //y 4
  31. let [m=2,n=3] = [undefined,null]
  32. //m 2
  33. //n null
  35. let {x,y=5} = {x:1}
  36. //x 1
  37. //y 5
  1. let {config:{method,url,data}}=response
  2. //相当于定义了4个变量