How do I identify sync APIs in Node.js?
Sync APIs often have names ending in Sync, like fs.readFileSync. Async APIs use callbacks or return promises, like fs.readFile with a callback, or fs.promises.readFile with a promise. The naming convention makes it easy to tell which is which.
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More FAQs in Sync vs Async Code in Node.js: Understanding the Difference
Sync code runs line by line, blocking the thread until each line completes. Async code initiates an operation, returns immediately, and processes the result via a callback when complete, letting the thread process other work.
Only for scripts, build tools, and startup code where blocking does not affect concurrent connections. Reading a config file at startup with fs.readFileSync is fine. Never use sync APIs in a request handler or any code that runs during request processing.
Always in request handlers and any code that runs during request processing. Use fs.readFile, not fs.readFileSync. This keeps the event loop free for other connections while the I/O completes.
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