Should I use sync or async APIs in Node.js?
Always prefer async APIs (like fs.readFile) in production code so you do not block the event loop. Sync APIs (like fs.readFileSync) are acceptable only for scripts, build tools, and startup code where blocking does not affect concurrent connections.
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More FAQs in Blocking vs Non-Blocking Code in Node.js: The Key Difference
Blocking code stops the main thread until it completes, preventing all other requests. Non-blocking code initiates an operation, returns immediately, and processes the result via a callback when complete, letting the thread handle other requests.
Because Node.js runs JavaScript on a single thread. One blocking operation stops all requests. This is the biggest performance concern in Node.js, and why the community emphasizes async APIs for all production code.
For scripts, build tools, and startup code where blocking does not affect concurrent connections. For example, reading a config file at startup with fs.readFileSync is fine. But never use sync APIs in a request handler or any code that runs during request processing.
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