TypeScript：从被装饰函数类型推导装饰器参数的类型安全约束

需求：实现一个类型安全的装饰器requiredParams，它接受的字符串数组元素必须是被装饰函数的request.params对象的键名。例如：

type Params = {id: string};

class Test {
  @requiredParams('id', 'name') // 此处应触发类型错误，因为`name`不是Params的键
  async handler(request: IRequest<void, Params>): Promise<void> {}
}

现有实现问题 #

当前的实现无法正确推断被装饰函数的TRequestParams类型，导致类型约束失效。原代码如下：

export function requiredParams<
    TRequestParams extends Record<string, string>,
    TRequiredRequestParams extends keyof TRequestParams & string
>(...requiredRequestParams: TRequiredRequestParams[]) {
    return function <
        TThis,
        TArguments extends Parameters<AsyncRequestHandler<any, TRequestParams, any>>,
        TReturn
    >(
        target: (this: TThis, ...args: TArguments) => Promise<TReturn>,
        context: ClassMethodDecoratorContext<TThis, (this: TThis, ...args: TArguments) => Promise<TReturn>>,
    ) {
        return async function(this: TThis, ...args: TArguments): Promise<TReturn> { 
            // 校验逻辑示例
            const params = args[0].params;
            for (const param of requiredRequestParams) {
                if (!(param in params)) {
                    throw new Error(`缺少必填参数：${param}`);
                }
            }
            return target.apply(this, args);
        };
    };
}

依赖的类型定义：

type AsyncRequestHandler<
    TBody = any,
    TParams = any,
    TQuery = any
> = (request: IRequest<TBody, TParams, TQuery>, response: IResponse, next?: Next) => Promise<void>;

interface IRequest<TBody = any, TParams = any, TQuery = any> {
    body: TBody;
    params: TParams;
    query: TQuery;
}

// 补充缺失的基础类型
interface IResponse {}
type Next = () => void;

修正后的实现 #

问题核心在于泛型推断顺序错误，应该让requiredParams从被装饰函数的参数中推断TRequestParams，而非提前声明。调整后的代码：

export function requiredParams<
    TRequired extends string[]
>(...required: TRequired) {
    return function <
        TThis,
        TReq extends IRequest<any, infer TParams, any>,
        TArgs extends [TReq, IResponse, Next?],
        TReturn
    >(
        target: (this: TThis, ...args: TArgs) => Promise<TReturn>,
        context: ClassMethodDecoratorContext<TThis, (this: TThis, ...args: TArgs) => Promise<TReturn>>,
    ) {
        // 约束传入的参数必须是TParams的键
        type AllowedParams = keyof TParams & string;
        const _required = required as readonly AllowedParams[];

        return async function(this: TThis, ...args: TArgs): Promise<TReturn> {
            const params = args[0].params;
            for (const param of _required) {
                if (!(param in params)) {
                    throw new Error(`缺少必填参数：${param}`);
                }
            }
            return target.apply(this, args);
        };
    };
}

关键改进点 #

1. 泛型推断顺序调整：让装饰器的返回函数先推断被装饰函数的TReq类型，再通过infer TParams提取出params的类型。
2. 类型约束后置：在返回函数内部通过AllowedParams约束传入的required参数必须是TParams的键，此时TypeScript会根据被装饰函数的request参数类型进行校验。
3. 类型断言兼容：使用as readonly AllowedParams[]确保类型校验通过，同时保留运行时的数组值。

验证效果 #

使用开头的测试代码，@requiredParams('id', 'name')会触发类型错误，提示'name'不能赋值给keyof Params类型，符合预期。

内容的提问来源于stack exchange，提问作者tdranv