TypeScript 中是否有类似于“keyof”的“valueof”？
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Is there a `valueof` similar to `keyof` in TypeScript?
提问by styfle
I want to be able to assign an object property to a value given a key and value as inputs yet still be able to determine the type of the value. It's a bit hard to explain so this code should reveal the problem:
我希望能够将一个对象属性分配给一个给定键和值作为输入的值，但仍然能够确定值的类型。解释起来有点困难，所以这段代码应该可以揭示问题：
type JWT = { id: string, token: string, expire: Date };
const obj: JWT = { id: 'abc123', token: 'tk01', expire: new Date(2018, 2, 14) };
function print(key: keyof JWT) {
switch (key) {
case 'id':
case 'token':
console.log(obj[key].toUpperCase());
break;
case 'expire':
console.log(obj[key].toISOString());
break;
}
}
function onChange(key: keyof JWT, value: any) {
switch (key) {
case 'id':
case 'token':
obj[key] = value + ' (assigned)';
break;
case 'expire':
obj[key] = value;
break;
}
}
print('id');
print('expire');
onChange('id', 'def456');
onChange('expire', new Date(2018, 3, 14));
print('id');
print('expire');
onChange('expire', 1337); // should fail here at compile time
print('expire'); // actually fails here at run time
I tried changing value: anyto value: valueof JWTbut that didn't work.
我尝试更改value: any为，value: valueof JWT但没有用。
Ideally, onChange('expire', 1337)would fail because 1337is not a Date type.
理想情况下，onChange('expire', 1337)会失败，因为1337不是 Date 类型。
How can I change value: anyto be the value of the given key?
如何更改value: any为给定键的值？
回答by jcalz
UPDATE: Looks like the question title attracts people looking for a union of all possible property value types, analogous to the way keyofgives you the union of all possible property key types. Let's help those people first. You can make a ValueOfanalogous to keyof, by using lookup typeswith keyof Tas the key, like so:
更新：看起来问题标题吸引人们寻找所有可能的属性值类型的联合，类似于keyof为您提供所有可能的属性键类型的联合的方式。让我们先帮助那些人。您可以通过使用带有作为键的查找类型来创建ValueOf类似于，如下所示：keyofkeyof T
type ValueOf<T> = T[keyof T];
which gives you
这给了你
type Foo = { a: string, b: number };
type ValueOfFoo = ValueOf<Foo>; // string | number
For the question as stated, you can use individual keys, narrower than keyof T, to extract just the value type you care about:
对于上述问题，您可以使用比 窄的单个键keyof T来提取您关心的值类型：
type sameAsString = Foo['a']; // lookup a in Foo
type sameAsNumber = Foo['b']; // lookup b in Foo
In order to make sure that the key/value pair "match up" properly in a function, you should use genericsas well as lookup types, like this:
为了确保键/值对在函数中正确“匹配”，您应该使用泛型以及查找类型，如下所示：
declare function onChange<K extends keyof JWT>(key: K, value: JWT[K]): void;
onChange('id', 'def456'); // okay
onChange('expire', new Date(2018, 3, 14)); // okay
onChange('expire', 1337); // error. 1337 not assignable to Date
The idea is that the keyparameter allows the compiler to infer the generic Kparameter. Then it requires that valuematches JWT[K], the lookup type you need.
这个想法是key参数允许编译器推断泛型K参数。然后它需要value匹配JWT[K]，您需要的查找类型。
Hope that helps; good luck!
希望有所帮助；祝你好运！
回答by Chris Kowalski
If anyone still looks for implementation of valueoffor any purposes, this is a one I came up with:
如果有人仍然valueof出于任何目的寻找实现，这是我想出的：
type valueof<T> = T[keyof T]
Usage:
用法：
type actions = {
a: {
type: 'Reset'
data: number
}
b: {
type: 'Apply'
data: string
}
}
type actionValues = valueof<actions>
Works as expected :) Returns an Union of all possible types
按预期工作:) 返回所有可能类型的联合
回答by Dima
There is another way to extract the union type of the object:
还有一种方法可以提取对象的联合类型：
const myObj = { a: 1, b: 'some_string' } as const;
type values = typeof myObj[keyof typeof myObj];
Result: 1 | "some_string"
结果： 1 | "some_string"
回答by Billy Chan
Thanks the existing answers which solve the problem perfectly. Just wanted to add up a lib has included this utility type, if you prefer to import this common one.
感谢完美解决问题的现有答案。只是想添加一个包含此实用程序类型的库，如果您更喜欢导入这个常用的。
https://github.com/piotrwitek/utility-types#valuestypet
https://github.com/piotrwitek/utility-types#valuestypet
import { ValuesType } from 'utility-types';
type Props = { name: string; age: number; visible: boolean };
// Expect: string | number | boolean
type PropsValues = ValuesType<Props>;
回答by Zheeeng
Try this:
尝试这个：
type ValueOf<T> = T extends any[] ? T[number] : T[keyof T]
It works on an array or a plain object.
它适用于数组或普通对象。
// type TEST1 = boolean | 42 | "heyhey"
type TEST1 = ValueOf<{ foo: 42, sort: 'heyhey', bool: boolean }>
// type TEST2 = 1 | 4 | 9 | "zzz..."
type TEST2 = ValueOf<[1, 4, 9, 'zzz...']>
回答by José Cabo
One-liner:
单线：
type ValueTypesOfPropFromMyCoolType = MyCoolType[keyof MyCoolType];
Example on a generic method:
泛型方法示例：
declare function doStuff<V extends MyCoolType[keyof MyCoolType]>(propertyName: keyof MyCoolType, value: V) => void;
