JavaScript's Promise Synchronous Processing

Promises in JavaScript are a solution to asynchronous programming that provide an easy way to handle the results of asynchronous operations. In JavaScript, Promise objects have two states: pending and fulfilled. When an asynchronous operation starts, the state of Promise is pending. If the asynchronous operation succeeds, the state of Promise will become fulfilled. If the asynchronous operation fails, the state of Promise will become rejected.

Promise synchronization can be achieved using async/await.

async function processPromise() {
  const result = await promise;
  console.log(result);
}

In the above code, we declare an async function processPromise and use await inside the function to wait for the result of the promise object. When the state of the promise object becomes fulfilled, the value of the await expression is assigned to the result variable.

It should be noted that async/await can only be used in functions.

Using async/await allows you to write synchronous style code in JavaScript without using callback functions and chaining calls. This makes the code much more concise and readable.

For example, if you want to perform three asynchronous operations, and you want to execute them sequentially, you can use async/await to write code:

async function processAll() {
  const result1 = await promise1;
  console.log(result1);
  const result2 = await promise2;
  console.log(result2);
  const result3 = await promise3;
  console.log(result3);
}

In the above code, we use three await expressions to wait for the results of the three promise objects, so that we can ensure that promise1, promise2, and promise3 are executed in order.

It should be noted that if the state of the promise object becomes rejected, the await expression will throw an error, and you can use the try/catch statement to catch the error.

async function processAll() {
  try {
    const result1 = await promise1;
    console.log(result1);
    const result2 = await promise2;
    console.log(result2);
    const result3 = await promise3;
    console.log(result3);
  } catch (error) {
    console.error(error);
  }
}

Another thing to note is that using async/await will make the code more concise, but it may also make the code more complicated. If your code has a lot of nested asynchronous operations, using async/await can make the code ugly.

Also, if your code has a lot of concurrent asynchronous operations, using async/await may make the code very slow. Because async/await is executed sequentially, if there are multiple concurrent asynchronous operations, they will be executed sequentially, which may cause performance problems.

To solve this problem, you can use the Promise.all method to execute multiple asynchronous operations concurrently

async function processAll() {
  const [result1, result2, result3] = await Promise.all([promise1, promise2, promise3]);
  console.log(result1, result2, result3);
}

When using async/await, it is necessary to consider the complexity and performance problems it may bring, and use the corresponding method to optimize the code where appropriate.

In addition, in order to solve the problem of async/await in concurrent processing, there is also a library called that async_hookscan be used to optimize code. It provides a more flexible way to handle concurrent asynchronous operations.

For example, you can use the async_hooks library to implement a concurrency limiter that allows you to limit the number of asynchronous operations that run concurrently.

const async_hooks = require('async_hooks');

class Limiter {
  constructor(concurrency) {
    this.concurrency = concurrency;
    this.active = 0;
    this.queue = [];
    this.hooks = async_hooks.createHook({
      init: (asyncId, type, triggerAsyncId, resource) => {
        if (this.active >= this.concurrency) {
          resource.emitBefore();
          this.queue.push(resource);
        } else {
          this.active++;
        }
      },
      before: (asyncId) => {
        if (this.queue.length > 0) {
          const resource = this.queue.shift();
          this.active++;
          resource.emitBefore();
        } else {
          this.active--;
        }
      },
    });
  }
  start() {
    this.hooks.enable();
  }
  stop() {
    this.hooks.disable();
  }
}

Use this concurrency limiter to limit the number of asynchronous operations running simultaneously, so that code performance can be guaranteed.

async_hooks is a very powerful tool that can help you optimize asynchronous code in JavaScript and solve concurrency problems. But it should be noted that its use requires certain experience and skills.