[英]Typescript generic union type
我正在尝试创建一个简单的开关 function ,它采用第一个参数,该参数必须是字符串和 object 的联合,它具有基于第一个参数联合的键并且可以返回任何值。
export const mySwitch = <T extends string>(value: T, possibilities: {[key in T]: any}): any => {
return possibilities[value];
};
典型用法是
let option: "val1" | "val2" | "val3" = "val1";
// should returns s1
// Impossible should be type-checked as an error since it's not part of the option union type
mySwitch(option, {val1: "s1", val2: "s2", val3: "s3", impossible: "impossible"});
出现我的问题是因为泛型类型T
必须是string
才能用作 object 键。 我不知道你怎么能告诉T
是string
的并集。
我试过T extends string
没有成功。
T extends string
版本似乎运行良好。 它不允许impossible
,但是您不想禁止它,因为如果参数永远不能具有该值,那么该选项将无用吗?:
export const mySwitch = <T extends string>(value: T, possibilities: {[key in T]: any}): any => {
return possibilities[value];
};
declare let option: "val1" | "val2" | "val3";
mySwitch(option, {val1: "s1", val2: "s2", val3: "s3", impossible: "impossible"});
如果您想允许额外的键,您可以单独声明案例 object (绕过多余的属性检查并允许您重用案例对象)
declare let option: "val1" | "val2" | "val3";
const casses = {val1: "s1", val2: "s2", val3: "s3", impossible: "impossible"}
mySwitch(option, casses);
或者你可以稍微改变你的类型,这样泛型类型参数就是 object,值将被键入为keyof T
:
export const mySwitch = <T>(value: keyof T, possibilities: T): any => {
return possibilities[value];
};
declare let option: "val1" | "val2" | "val3";
mySwitch(option, {val1: "s1", val2: "s2", val3: "s3", impossible: "impossible"});
另外一个更好的选择是保留案例 object 的类型,而不是使用any
:
export const mySwitch = <T, K extends keyof T>(value: K, possibilities: T): T[K] => {
return possibilities[value];
};
declare let option: "val1" | "val2" | "val3";
mySwitch(option, {val1: 1, val2: "s2", val3: "s3", impossible: false}); // returns string | number
编辑:
如果联合中不存在可能性,要保留正确的返回类型和错误,您可以使用以下命令:
const mySwitch = <T extends Record<K, any>, K extends string>(value: K, possibilities: T & Record<Exclude<keyof T, K>, never>): any => {
return possibilities[value];
};
let option: "val1" | "val2" | "val3" = (["val1", "val2", "val3"] as const)[Math.round(Math.random() * 2)]
mySwitch(option, {val1: "s1", val2: "s2", val3: "s3" });
mySwitch(option, {val1: "s1", val2: "s2", val3: "s3", impossible: "" }); //err on impossible
请注意,因为 typescript 进行控制流分析,您需要确保option
不仅仅是类型作为您分配的实际常量,而不是您指定的类型注释
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