What Is The Best Way To Limit Concurrency When Using Es6s Promise All

When working with asynchronous JavaScript operations, you may find yourself in situations where you need to limit the concurrency of multiple promises running concurrently. One common method to handle this scenario is by using ES6's `Promise.all` with a concurrency limiter. Let's dive into how you can achieve this efficiently in your code.

First things first, let's understand what `Promise.all` does. It's a handy method that takes an array of promises and returns a new promise that resolves once all the promises in the array have resolved, or rejects as soon as one of the promises in the array rejects.

To limit the concurrency when using `Promise.all`, we can implement our own concurrency limiter. Here's a simple example of how you can achieve this:

function limitConcurrency(promises, limit) {
  const executing = [];
  const results = [];
  
  function runPromises() {
    if (promises.length === 0 && executing.length === 0) {
      // All promises have been executed
      return Promise.resolve(results);
    }
    
    while (executing.length  0) {
      const promise = promises.shift();
      const execution = promise().then(result => {
        results.push(result);
        executing.splice(executing.indexOf(execution), 1);
      });
      
      executing.push(execution);
    }
    
    return Promise.race(executing).then(() => runPromises());
  }

  return runPromises();
}

const promises = [promise1, promise2, promise3, /* add your promises here */];
const concurrencyLimit = 2; // Set your desired concurrency limit

limitConcurrency(promises, concurrencyLimit).then(results => {
  console.log(results);
});

In the code snippet above, the `limitConcurrency` function takes two parameters: an array of promises and a limit specifying how many promises should run concurrently. It creates a queue of promises that are currently executing and another for storing the results of the promises.

The `runPromises` function continues to execute promises from the input array as long as the concurrency limit allows it, using `Promise.race` to wait for any of the promises to resolve. Once a promise completes, it removes it from the executing queue and pushes the result into the results array.

By implementing a custom concurrency limiter like this, you can control how many promises are executing simultaneously when using `Promise.all`. This approach can be particularly useful in scenarios where you want to avoid overwhelming external APIs or server resources by limiting concurrent requests.

In conclusion, when you need to limit concurrency while using ES6's `Promise.all`, consider implementing a custom concurrency limiter like the one demonstrated above. This approach gives you more control over how many promises are running concurrently and can help optimize the performance of your asynchronous operations.