Compare Promise.all, Promise.any, and Promise.allSettled

Scenario

You are designing an API orchestration layer that fetches data concurrently from multiple independent microservices. To ensure reliability and optimal latency, you must evaluate JavaScript's built-in promise concurrency combinators: Promise.all, Promise.any, and Promise.allSettled.

Requirements

1. Comparative Analysis

Provide a technical comparison of the three methods across the following dimensions:

• Fulfillment Condition: When does the returned promise resolve?
• Rejection Condition: When does the returned promise reject, and what is the rejection value?
• Result Structure: What does the resolved value look like?
• Result Ordering: Does the result maintain the order of the input iterable or the order of completion?
• Empty Iterables: How does each method behave when passed an empty iterable (e.g., [])?

2. Failure and Lifecycle Considerations

Explain how each method handles:

• Short-circuiting vs. Request Cancellation: If Promise.all rejections or Promise.any fulfillments short-circuit, what happens to the remaining pending promises? Does JavaScript cancel underlying network I/O?
• Error Retention: How do errors in rejected promises manifest, and how can you avoid unhandled promise rejection warnings when one promise rejects before others complete?

3. Scenario Selection

For each of the following use cases, select the most appropriate combinator and justify your choice:

1. Aggregated Dashboard: You need to load 5 independent widgets. If one widget fails, the remaining 4 widgets must still display their content, and the failed widget should show an error state.
2. Critical Transaction Check: A transaction requires validation from a fraud detection service, an inventory service, and a balance service. If any service denies or fails, the transaction must abort immediately.
3. Multi-Region Mirror: You request a static configuration file from three geographically distributed CDNs. You only need the fastest successful response to proceed.