如何检查字符串以将其范围缩小到枚举中的字符串，并让TypeScript识别它（节点示例）？

Question

Using basic node.js, in http.createServer I am taking the request , and taking the path from the request. I have a router object, keyed by paths, with values being handler functions.

// index.ts

export interface DataObject {
  queryStringObject: ParsedUrlQuery;
  method: string;
  headers: IncomingHttpHeaders;
  payload: any;
}

export type HandlerCallback = (statusCode: number, payload?: Object) => any;

export type Handler = (data: DataObject, callback: HandlerCallback) => any;

interface Router {
  ping: Handler;
  users: Handler;
  adminUsers: Handler;
  tokens: Handler;
}

// Define request router
const router: Router = {
  ping: handlers.ping,
  users: handlers.users,
  adminUsers: handlers.adminUsers,
  tokens: handlers.tokens,
};

export enum RestMethods {
  get = 'get',
  post = 'post',
  put = 'put',
  delete = 'delete',
}

export type RestHandler = {
  [method in RestMethods]: Handler;
};

Then, I have the file where handlers is defined:

interface Handlers {
  users: Handler;
  adminUsers: Handler;
  tokens: Handler;
  notFound: Handler;
  ping: Handler;
}

interface SubHandlers {
  users: RestHandler;
  adminUsers: RestHandler;
  tokens: RestHandler;
}

export const handlers: Handlers = {
  users: (data, callback) => {
    const acceptableMethods = ['get', 'post', 'put', 'delete'];
    if (acceptableMethods.indexOf(data.method) !== -1) {
      subHandlers.users[data.method](data, callback);
    }
  },
(...)

I get a typescript error on the line subhandlers.users[data.method]...

Error:(62, 14) TS7053: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Router'. No index signature with a parameter of type 'string' was found on type 'Router'.

data.method is a string because DataObject defines it that way (an incoming request could have anything in method ), and subhandlers.users is of type RestHandler , which is an object with keys in the enum RestMethods . But I am doing the check acceptableMethods.indexOf(data.method) !== -1 ... How can I make this check so that Typescript can realize that data.method has been restricted to only the strings that are in the enum?

Answer 1

There are two issues here; one is that acceptableMethods is inferred to be of type string[] , whereas you want the compiler to keep track of the string literal values it contains; you can fix that easily enough with a const assertion as in:

const acceptableMethods = ["get", "post", "put", "delete"] as const;

The second issue is that TypeScript does not understand or expect that, given an array array of type T[] and a value val , checking array.indexOf(val) !== -1 should narrow val to type T . Luckily you can get this behavior by making a user-defined type guard function :

function arrayIncludes<T>(
  arr: ReadonlyArray<T>,
  val: any
): val is T {
  return arr.indexOf(val) !== -1;
}

Now you can use arrayIncludes() in place of the indexOf check, and it should work for you:

export const handlers: Handlers = {
  users: (data, callback) => {
    const acceptableMethods = ["get", "post", "put", "delete"] as const;
    if (arrayIncludes(acceptableMethods, data.method)) {
      subHandlers.users[data.method](data, callback); // okay
    }
  }
};

Does that help? Good luck!

Link to code