How to Execute Promises in Sequence: A Comprehensive Guide to Synchronization Techniques

How to Synchronize a Sequence of Promises

In this detailed article, we'll provide comprehensive techniques to synchronize a sequence of promises, ensuring that they are resolved in a strict and sequential manner.

The Problem

You have an array of promise objects ([p1, p2, p3, ...]) that need to be resolved one after the other. Resolving a subsequent promise requires the completion of the preceding one. Additionally, if any promise in the sequence is rejected, the entire chain should be rejected without attempting to resolve further promises.

Solution Overview

We'll cover several solutions, ranging from manual iteration to leveraging Promise library features like Promise.map() and async/await. Each approach will clearly illustrate its implementation and provide practical examples.

Manual Iteration

Approach 1: Using a While Loop

function sequence(arr) {
  let i = 0;

  const next = () => {
    if (i < arr.length) {
      arr[i++].then(next);
    }
  };
  next();
}

Approach 2: Using a For Loop

function sequence(arr) {
  for (let i = 0; i < arr.length; i++) {
    await arr[i];
  }
}

Promise Library Solutions

Approach 3: Using Promise.mapSeries (Bluebird.js)

Promise.mapSeries(arr, (p) => p).then((results) => {
  // Process final results
});

Approach 4: Using Promise.mapSeries with Delay (Bluebird.js)

Promise.mapSeries(arr, (p) => delay(t, p)).then((results) => {
  // Process final results
});

function delay(t, v) {
  return new Promise((resolve) => {
    setTimeout(resolve.bind(null, v), t);
  });
}

Async/Await Solution

async function sequence(arr) {
  const results = [];
  for (let i = 0; i < arr.length; i++) {
    results.push(await arr[i]);
  }
  return results;
}

Caveats: Handling Rejections

It's important to note that in these solutions, if any promise in the sequence is rejected, the entire chain is interrupted, and the rejection is propagated. If you wish to handle rejections differently or want more control over the process, you need to modify the code to suit your specific requirements.

Conclusion

We've explored several approaches for synchronizing a sequence of promises, providing both manual and library-based solutions. The choice of approach depends on your specific requirements and the context of your application.

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