The answer
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.).
How to Create a Promise in JavaScript
When does the promise executor run in JavaScript?
More FAQs in How to Create a Promise 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).
Return new Promise((resolve, reject) => { callbackApi(args, (err, result) => { if (err) reject(err); else resolve(result); }); }). The callback follows the error-first convention.
The promise is rejected with the thrown error. The Promise constructor catches thrown errors automatically. You do not need to try/catch inside the executor.
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