Javascript - How to apply a method always before executing another method via a proxy?

kabdik
August 21, 2023
235 views
2 votes
3 Answers

I have the following code where I want to invoke the instance method connect before proceeding with the invocation of every other instance method of the TelegramClient class. How can this be achieved by making use of Proxy. As of now the connect method does not execute.

class TelegramClient {
  async connect() {
    console.log("Connecting to Telegram...");
    // Simulate some asynchronous work
    await new Promise(resolve => setTimeout(resolve, 1000));
    console.log("Connected!");
  }

  sendMessage(message: string) {
    console.log(`Sending message: ${message}`);
  }
}

const telegramClient = new TelegramClient();

// Create a Proxy for the Telegram client
const proxiedTelegramClient = new Proxy(telegramClient, {
  async apply(target, thisArg, argumentsList) {
    await target.connect(); // Connect to Telegram before invoking the method
    const result = Reflect.apply(target, thisArg, argumentsList);
    return result;
  },
});

proxiedTelegramClient.sendMessage("Hello, Telegram!");

this is my code. So the output that I want is:

Connecting to Telegram...
Connected!
Sending message: Hello, Telegram!

Answers

this can be resolved using a "get" instead of "apply" handler

//... previous code before the proxiedTelegramClient
// Create a Proxy for the Telegram client
const proxiedTelegramClient = new Proxy(telegramClient, {
  get(target, property) {
    if (property === 'sendMessage') {
      return async function (...args) {
        await target.connect(); // Connect to Telegram before sending the message
        return Reflect.apply(target[property], target, args);
      };
    }
    return target[property];
  },
});

but if you want to keep code as it is, you can mak use of

class TelegramClient {
  async connect() {
    console.log("Connecting to Telegram...");
    // Simulate some asynchronous work
    await new Promise(resolve => setTimeout(resolve, 1000));
    console.log("Connected!");
  }

  async sendMessage(message: string) {
    await this.connect(); // Connect to Telegram before sending the message
    console.log(`Sending message: ${message}`);
  }
}

const telegramClient = new TelegramClient();

// Create a Proxy for the Telegram client
const proxiedTelegramClient = new Proxy(telegramClient, {
  async apply(target, thisArg, argumentsList) {
    return Reflect.apply(target, thisArg, argumentsList);
  },
});

(async () => {
  await proxiedTelegramClient.sendMessage("Hello, Telegram!");
})();

hope it helps

The main problem here is that apply is not async, marking async won’t change that.

Rather than trying to intercept the sendMessage method, another option is to auto-wrap the function inside the get.

Below is an example, I’ve also use a Set to make sure during get the method does not get wrapped again. Also added a connected property, as sending another message shouldn’t require another connect.

const connected = new WeakSet();

class TelegramClient {

  async connect() {
    if (connected.has(this)) return;
    connected.add(this);
    console.log("Connecting to Telegram...");
    // Simulate some asynchronous work
    await new Promise(resolve => setTimeout(resolve, 1000));
    console.log("Connected!");
  }

  async sendMessage(message) {    
    console.log(`Sending message: ${message}`);
  }
}

const telegramClient = new TelegramClient();

const wrapped = new Set();

const handler = {
  get(target, prop, receiver) {
    if (['connect', 'connected'].includes(prop) || wrapped.has(prop) ) {
      return Reflect.get(...arguments);
    }
    const fn = target[prop];
    function wrap() {
      return target.connect().then(() => {
        return fn(...arguments);
      });
    }
    wrapped.add(prop);
    target[prop] = wrap;
    return target[prop];
  }
}

// Create a Proxy for the Telegram client
const proxiedTelegramClient = new Proxy(telegramClient, handler);

(async () => {
  await proxiedTelegramClient.sendMessage("Hello, Telegram!");
  await proxiedTelegramClient.sendMessage("Another message.");
})();

A viable approach which does not utilize a proxy was to use a generic implementation of an async around method-modifier. The latter can be seen as a specialized case of function-wrapping.

Such a modifier accepts two functions, proceed and handler as well as a target-object as its 3 parameters. It does return an async function which again is going to return the awaited result of the (assumed async) handler (callback) function. The latter does get invoked within the context of the (optionally) provided target while also getting passed the proceed-function, its own handler-reference and the modified function’s arguments-array.

Thus, based on such a modifier, the OP could achieve the expected behavior by modifying e.g. a client instance’s sendMessage-method like this …

// client instantiation.
const telegramClient = new TelegramClient;

// client's handler modification.
telegramClient.sendMessage = asyncAround(
  telegramClient.sendMessage,
  connectClientBeforeProceed,
  telegramClient,
);

… where connectClientBeforeProceed is the handler function which implements exactly what the OP is looking for …

"… connect[ing the client] before proceeding with the invocation of every other instance method of the TelegramClient class"

… Example code …

// implementation of the client specific async `around`-handler.
async function connectClientBeforeProceed(proceed, handler, args) {
  const client = this;

  // ... always connect client ...
  await client.connect();

  // ... before proceeding with any other method.
  return (
    await proceed.apply(client, args)
  );
}

// client instantiation.
const telegramClient = new TelegramClient;

// client's method-modification.
telegramClient.sendMessage = asyncAround(
  telegramClient.sendMessage,
  connectClientBeforeProceed,
  telegramClient,
);

// client's modified handler invocation.
telegramClient.sendMessage("Hello, Telegram!");


// expected logging ..
//
//  - Connecting to Telegram...
//  - Connected!
//  - Sending message: Hello, Telegram!

.as-console-wrapper { min-height: 100%!important; top: 0; }

<script>

// client implementation.
class TelegramClient {
  async connect() {
    console.log("Connecting to Telegram...");

    // Simulate some asynchronous work
    await new Promise(resolve => setTimeout(resolve, 1000));

    console.log("Connected!");
  }
  sendMessage(message) {
    console.log(`Sending message: ${ message }`);
  }
}

// implementation of an async `around` modifier.
function asyncAround(proceed, handler, target) {
  return async function (...args) {

    return (
      await handler.call(target ?? null, proceed, handler, args)
    );
  };
}

</script>

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Javascript – How to apply a method always before executing another method via a proxy?

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