Promise all() vs allSettled()

In JavaScript's asynchronous world, managing multiple promises efficiently is crucial for optimizing web application performance and user experience. Two important methods for handling collections of promises are Promise.all() and Promise.allSettled(). Each serves a unique purpose in the asynchronous workflow, tailored to different requirements and scenarios.

Promise.all()

Promise.all() is a method used when working with multiple promises concurrently. It takes an iterable of promises as an input and returns a single Promise that:

• Fulfills when all input promises have fulfilled. In this case, the returned promise resolves with an array containing the values of the input promises in the same order.
• Rejects as soon as one of the input promises rejects. The returned promise is rejected with the reason of the first promise that rejects.

Use Case: Promise.all() is ideal when all asynchronous operations need to succeed for the program to proceed. It's commonly used in scenarios where the failure of any single operation means the collective task cannot be completed or doesn't make sense to continue.

Promise.allSettled()

Introduced in ES2020, Promise.allSettled() differs from Promise.all() by how it handles the resolution and rejection of input promises. It also takes an iterable of promises but returns a promise that:

• Always fulfills after all input promises have settled (either fulfilled or rejected). The resolved value is an array of objects, each describing the outcome of each promise with a status of either "fulfilled" or "rejected" and a corresponding value or reason.

Use Case: Promise.allSettled() is useful when you need to perform multiple operations independently and want to know the outcome of each, regardless of whether they succeed or fail. This method is suitable for scenarios where partial success is acceptable, or you need to perform cleanup or follow-up actions based on individual results.

Comparison: While Promise.all() is strict in requiring all promises to fulfill, Promise.allSettled() offers a more flexible approach, allowing developers to handle both successes and failures collectively. The choice between the two depends on whether the completion of your operation requires all tasks to succeed or can tolerate some failures.

Understanding the distinctions and appropriate use cases for Promise.all() and Promise.allSettled() enables developers to write more robust, efficient, and resilient asynchronous code, adeptly managing multiple concurrent operations.