What makes JavaScript able to handle async without blocking?
Web APIs handle the work outside the engine, queues store callbacks until the stack is empty, and the event loop moves callbacks to the stack at the right time. The engine itself stays synchronous and non-blocking.
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More FAQs in How JavaScript Handles Asynchronous Operations
Through the runtime environment. Web APIs (browser) or libuv (Node.js) handle async operations outside the engine. When done, callbacks go to queues. The event loop moves callbacks to the call stack when it is empty. The engine itself stays synchronous.
Browser-provided async operations: setTimeout, fetch, DOM events, geolocation, etc. They run outside the JS engine. When the operation completes, the callback is pushed to the microtask or macrotask queue for the event loop to pick up.
Synchronous at the language level (one call stack, one statement at a time). The runtime (Web APIs, queues, event loop) provides asynchronous behavior. The engine itself never runs two things at once.
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