How to Implement a Completion Callback for Asynchronous forEach Loops?

Asynchronous forEach Callbacks: Implementing a Completion Callback

In scenarios where an asynchronous loop like forEach is employed, the need often arises to trigger a callback once all asynchronous operations have completed. This guide presents several approaches to achieve this functionality.

1. Utilizing a Simple Counter

This technique employs a counter that increments every time an item is processed. When the counter reaches the total number of items, the completion callback is invoked.

function callback () { console.log('all done'); }

var itemsProcessed = 0;

[1, 2, 3].forEach((item, index, array) => {
  asyncFunction(item, () => {
    itemsProcessed++;
    if(itemsProcessed === array.length) {
      callback();
    }
  });
});

2. Leveraging ES6 Promises

Promises enable the creation of a sequential promise chain, ensuring synchronous execution of asynchronous tasks.

function asyncFunction (item, cb) {
  setTimeout(() => {
    console.log('done with', item);
    cb();
  }, 100);
}

let requests = [1, 2, 3].reduce((promiseChain, item) => {
    return promiseChain.then(() => new Promise((resolve) => {
      asyncFunction(item, resolve);
    }));
}, Promise.resolve());

requests.then(() => console.log('done'))

3. Employing an Asynchronous Library

Libraries like "async" provide mechanisms for expressing the desired functionality. The specific implementation will vary depending on the library chosen.

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