What guarantees do promises provide in JavaScript?
A promise settles (resolves or rejects) exactly once. .then callbacks run asynchronously (as microtasks), never synchronously. The callback receives the resolved value or the rejection reason. These guarantees solve inversion of control.
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More FAQs in Inversion of Control in JavaScript Callbacks
When you pass a callback to a function, you give up control of when and how it is called. The function might call it multiple times, not at all, too early, or with wrong arguments. You are trusting the function to behave correctly.
Promises guarantee once-only settlement (resolve or reject exactly once), async execution (.then callbacks run as microtasks), and value-or-error delivery. You trust the promise spec, not the function calling the callback.
The function might call the callback multiple times (duplicate effects), zero times (page hangs), too early (before setup is done), with wrong arguments (undefined), or synchronously when you expected async.
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