The answer
Promise.all runs multiple promises in parallel and waits for all to complete. Results are in order. If any rejects, the whole thing rejects. With callbacks, you need a manual counter.
How Promises Solve Callback Problems in JavaScript
How do promises help with parallel async operations in JavaScript?
More FAQs in How Promises Solve Callback Problems in JavaScript
Promises use .then chains instead of nested callbacks. Each .then returns a new promise, so you can chain flatly instead of nesting. This eliminates the pyramid of doom.
A promise settles exactly once. resolve/reject can only be called once. .then callbacks always run asynchronously as microtasks. You trust the promise spec, not the function calling the callback.
One .catch at the end of a .then chain handles errors from anywhere in the chain. With callbacks, each callback must check if (err) and propagate, which is repetitive.
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