是什么

可遍历类型的所有属性，keyof 将提取属性的的字面量类型，然后构成一个联合类型。 keyof 是引索类型查询操作符。keyof T;

keyoof 返回联合类型

  1. interface Person {
  2.   name: string;
  3.   age: number;
  4.   isChina: boolean;
  5. }
  7. interface Map<T> {
  8.   [K: string]: T;
  9. }
  11. class PersonClass {
  12.   name = 'stone';
  13.   getName(){
  14.     return this.name;
  15.   }
  16. }
  18. let person: keyof Person;
  19. // let person: "name" | "age" | "isChina"
  21. let person1: keyof Person[];
  22. // let person1: number | "length" | "toString" | "toLocaleString" | "pop" | "push" | "concat" | "join" | "reverse" | "shift" | "slice" | "sort" | "splice" | "unshift" | "indexOf" | "lastIndexOf" | ... 6 more ... | "reduceRight"
  24. let person2: keyof Map<string>;
  25. // let person2: string | number
  27. let person3: keyof PersonClass;
  28. // let person3: "name" | "getName"
  29. person3 = 'name';
  30. person3 = 'getName';
  31. // person3 = 'age'; // 不能将类型“"age"”分配给类型“"name" | "getName"”。ts(2322)
  33. let k1: keyof string;
  34. // let k1: number | "length" | "toString" | "concat" | "slice" | "indexOf" | "lastIndexOf" | "charAt" | "charCodeAt" | "localeCompare" | "match" | "replace" | "search" | "split" | "substring" | ... 6 more ... | "valueOf"
  35. let k2: keyof number;
  36. // let k2: "toString" | "toLocaleString" | "valueOf" | "toFixed" | "toExponential" | "toPrecision"
  37. let k3: keyof boolean;
  38. // let k3: "valueOf"
  39. let k4: keyof null;
  40. // let k4: never
  41. let k5: keyof undefined;
  42. // let k5: never

keyof 保证属性的安全访问

  1. type Todo = {
  2.   name: string;
  3.   age: number;
  4.   isChina: boolean;
  5. };
  7. let todo: Todo = {
  8.   name: 'stone',
  9.   age: 18,
  10.   isChina: true
  11. };
  13. function prop<T extends object, K extends keyof T>(obj: T, key: K) {
  14.   return obj[key];
  15. }
  17. let l_name = prop(todo, 'name');
  18. // let l_name: string
  19. let l_age = prop(todo, 'age');
  20. // let l_age: number
  21. let l_ischina = prop(todo, 'isChina');
  22. // let l_ischina: boolean
  23. // let l_address = prop(todo, 'address');
  24. // 类型“"address"”的参数不能赋给类型“"name" | "age" | "isChina"”的参数。ts(2345)
  1. enum Currency {
  2.   CNY = 6,
  3.   EUR = 8,
  4.   USD = 10,
  5. };
  7. const CurrencyName = {
  8.   [Currency.CNY]: '人民币',
  9.   [Currency.EUR]: '英镑',
  10.   [Currency.USD]: '美元',
  11. };
  13. function getCurrencyName<T, K extends keyof T>(map: T, key: K): T[K] {
  14.   return map[key];
  15. }
  17. let name_1 = getCurrencyName(CurrencyName, Currency.CNY);
  18. let name_2 = getCurrencyName(CurrencyName, Currency.EUR);
  19. let name_3 = getCurrencyName(CurrencyName, Currency.USD);