I have the following code on typescript playground and a few questions come up that I am not sure how to get working

class PathInfo {
    functionName: string;
    httpPath: string;
    httpMethod: string;

    constructor(functionName: string, httpPath: string, httpMethod: string) {
        this.functionName = functionName;
        this.httpPath = httpPath;
        this.httpMethod = httpMethod;
    }

    toString(): string {
        return "PathInfo[" this.functionName "," this.httpPath "," this.httpMethod "]";
    }
}

class AuthRequest {}
class AuthResponse {}
class LoginRequest {}
class LoginResponse {}

const path: any = (thePath: string, type: any) => {
    return (target: Function, memberName: string, propertyDescriptor: PropertyDescriptor) => {
        const pathMeta = new PathInfo(memberName, path, type);

        Object.defineProperty(target, memberName 'pathInfo', {
            value: pathMeta,
            writable: false
        });

        //How do I access the stored pathMeta
        //console.log("target=" target.pathInfo);
        console.log("member=" memberName);
        console.log("props=" propertyDescriptor);
    }
}

class AuthApiImpl {
    @path("/authenticate", AuthResponse)
    authenticate(request: AuthRequest): Promise<AuthResponse> {
        throw new Error("all this is generated by factory.createApiImpl");
    }
    @path("/login", LoginResponse)
    login(request: LoginRequest): Promise<LoginResponse> {
        throw new Error("all this is generated by factory.createApiImpl");
    }
};

function printMethods(obj: any) {
    console.log("starting to print methods");

  for (var id in obj) {
      console.log("id=" id);
    try {
      //How do I access the stored pathMeta here FOR EACH METHOD ->
      //console.log("target=" target.pathInfo);
      if (typeof(obj[id]) == "function") {
          console.log(id ":" obj[id].toString());
      }
    } catch (err) {
      console.log(id   ": inaccessible" err);
    }
  }

}

console.log("starting to run")

const temp = new AuthApiImpl();
printMethods(temp);

console.log("done")

line 64-65, how to read the property that I set
line 40-41, how to read the property that I set
line 58-74, why is this not printing any functions? I want to print all functions and I do NOT want to print properties (just functions)
line 33, Can I access the class name at this point?
line 35, I thought target was a function and would be authorize, then login, BUT if I define the property as JUST 'pathInfo', I get an error that the property is already defined on the target(This implies the target is the class not the function?). I am so confused.

Terribly sorry as I try to focus on a single question, but this one test of writing decorators has given me more questions than answers as I delve into the typescript world.

How can I tweak the code to play more here?

A goal here is as developers define the APIs of other microservices, I can capture a bunch of meta information and store it SOMEWHERE I can use later in startup code. I do not care where I store that really, but just need a clean way of knowing the class I want to extend, the methods, the return types, the http path, etc.

CodePudding user response：

How to get methods of a class

You still can't grab the method names even if you remove the decorator. This isn't a TypeScript specific question.

You need to get the properties of the prototype, not just the object itself.

function printMethods(obj: any) {
    console.log("starting to print methods");

    const objProto = Object.getPrototypeOf(obj);
    console.log(Object.getOwnPropertyNames(objProto));
}

How to access class names

Don't think this is possible with decorators at the moment, but it should be straightforward to just pass in your class name as a string.

Open issue on GitHub: https://github.com/microsoft/TypeScript/issues/1579

"property is already defined on the target"

Notice if you run the code above you get the following in console.log:

["constructor", "authenticate", "login", "authenticatepathInfo", "loginpathInfo"]

I also want to point out that if you don't even initialize an instance of the class, you'll still get the same error.

CodePudding user response：

I want to read this meta data in nodejs and use that to dynamically create a client implementing the api. Basically, developers never have to write clients and only write the api and the implementation is generated for them.

If I were to do that, I'd probably not use decorators, but mapped types:

// library code
interface ApiMethodInfo {
    httpPath: string;
    httpMethod: string;
}

type ApiInfo<S extends object> = Record<keyof S, ApiMethodInfo>;
type Client<S extends object> = {[key in keyof S]: S[key] extends (req: infer Req) => infer Res ? (req: Req) => Promise<Res> : never};

function generateClient<S extends object>(apiInfo: ApiInfo<S>): Client<S> {
    const client = {} as Client<S>;
    for (const key in apiInfo) {
        const info = apiInfo[key as keyof S];
        client[key] = ((param: any) => invokeApi(info, param)) as any;
    }
    return client;
}

// application code

interface AuthRequest {}
interface AuthResponse {}
interface LoginRequest {
    username: string,
    password: string,
}
interface LoginResponse {}

interface MyServer {
    authenticate(request: AuthRequest): AuthResponse;
    login(request: LoginRequest): LoginResponse;
}

const myApiInfo: ApiInfo<MyServer> = { // compiler verifies that all methods of MyServer are described here
    authenticate: {
        httpPath: '/authenticate',
        httpMethod: 'POST'
    },
    login: {
        httpPath: '/login',
        httpMethod: 'POST'
    }
}

const myClient = generateClient(myApiInfo); // compiler derives the method signatures from the server implementation
const username = "joe";
const password = "secret";
const response = myClient.login({username, password}); // ... and can therefore check that this call is properly typed

(To understand how these type definitions work, you may want to read the section Creating Types from Types in the TypeScript Handbook)

The weakness of this approach is that while the compiler can derive the client signatures from the server signatures, it will not copy any JSDoc, so client devs can not easily access the API documentation.

In the above code, I chose to specify the metadata in a separate object rather than decorators so the compiler can check exhaustiveness (decorators are always optional; the compiler can not be instructed to require their presence), and because decorators are an experimental language feature that may still change in future releases of the language.

It is entirely possible to populate such a metadata object using decorators if that's what you prefer. Here's what that would look like:


// library code
interface ApiMethodInfo {
    httpPath: string;
    httpMethod: string;
}

const apiMethodInfo = Symbol("apiMethodInfo");

function api(info: ApiMethodInfo) {
    return function (target: any, propertyKey: string) {
        target[apiMethodInfo] = target[apiMethodInfo] || {};
        target[apiMethodInfo][propertyKey] = info;
    }
}

type ApiInfo<S extends object> = Record<keyof S, ApiMethodInfo>;
type Client<S extends object> = {[key in keyof S]: S[key] extends (req: infer Req) => infer Res ? (req: Req) => Promise<Res> : never};

function invokeApi(info: ApiMethodInfo, param: any) {
    console.log(info, param);
}

function generateClient<S extends object>(serverClass: new() => S): Client<S> {
    const infos = serverClass.prototype[apiMethodInfo]; // a decorator's target is the constructor function's prototype
    const client = {} as Client<S>;
    for (const key in infos) { // won't encounter apiMethodInfo because Symbol properties are not enumerable
        const info = infos[key];
        client[key as keyof S] = ((param: any) => invokeApi(info, param)) as any;
    }
    return client;
}

// application code

interface AuthRequest {}
interface AuthResponse {}
interface LoginRequest {
    username: string,
    password: string,
}
interface LoginResponse {}

class MyServer {
    @api({
        httpPath: '/authenticate',
        httpMethod: 'POST'
    })
    authenticate(request: AuthRequest): AuthResponse {
        throw new Error("Not implemented yet");
    }

    @api({
        httpPath: '/login',
        httpMethod: 'POST'
    })
    login(request: LoginRequest): LoginResponse {
        throw new Error("Not implemented yet");
    }
}

const myClient = generateClient(MyServer); // compiler derives the method signatures from the server implementation
const username = "joe";
const password = "secret";
const response = myClient.login({username, password}); // ... and can therefore check that this call is properly typed

Notice how using a Symbol prevents name collisions, and ensures that other code doesn't see this property (unless they look for that particular Symbol), and therefore can not be tripped up by its unexpected presence.

Also notice how MyServer, at runtime, contains the constructor of the class, whose prototype holds the declared instance methods, and it being passed as target to any decorators thereof.

General Advice

May I conclude with some advice for the recovering Java programmer? ;-)

EcmaScript is not Java. While the syntax may look similar, EcmaScript has many useful features Java does not, which often allow writing far less code. For instance, if you need a DTO, it is wholly unnecessary to declare a class with a constructor manually copying each parameter into a property. You can simply declare an interface instead, and create the object using an object literal. I recommend looking through the Modern JavaScript Tutorial to familiarize yourself with these useful language features.

Also, some features behave differently in EcmaScript. In particular, the distinction between class and interface is quite different: Classes are for inheriting methods from a prototype, interfaces for passing data around. It's quite nonsensical to declare a class for a Response that will be deserialized from JSON, because prototypes don't survive serialization.