When does the promise executor function run in JavaScript?
Immediately and synchronously, when the promise is created. The executor is not deferred. But resolve/reject can be called asynchronously (inside a setTimeout, event handler, etc.).
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More FAQs in Creating Promises in JavaScript
Use new Promise((resolve, reject) => { ... }). The executor runs immediately. Call resolve(value) to fulfill or reject(reason) to reject. For already-settled promises, use Promise.resolve(value) or Promise.reject(reason).
Creating an already-fulfilled promise. Use it when you have a value but need a promise (e.g., returning from a function that sometimes does sync work). Example: if (cached) return Promise.resolve(cached); else return fetch(url);
The outer promise adopts the state of the inner promise. If the inner promise fulfills, the outer fulfills with the same value. If the inner rejects, the outer rejects with the same reason. The outer waits for the inner.
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