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Mechanics Behind Async Iterators and For-Await-Of in JavaScript
- JavaScript treats asynchronous values and iterable behavior as separate until used with for-await-of loop, where they come together.
- An object is iterable when it has a method tied to Symbol.iterator that returns an object with a .next() method.
- Async iterators are similar, but the .next() method returns a Promise, making the loop wait for Promise resolution.
- for-await-of loop uses Symbol.asyncIterator to handle Promises and values with done flags.
- Async generators combine yielding values like regular generators with waiting on Promises, allowing for pausing and waiting during execution.
- The for-await-of loop waits for each yield from an async generator, allowing values to be processed as they arrive.
- JavaScript engine turns async generators into paused state machines, enabling natural pause and resume functionality without manual handling.
- Code within the for-await-of loop is paused using microtasks while waiting for Promises, ensuring asynchronous loops remain responsive.
- for-await-of loop throws exceptions for rejected Promises, and the entire loop structure should be inside a try block to catch errors.
- If a loop exits early, the engine checks for a .return() method in the iterator to clean up resources before finishing.
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