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Pascual Vila

Frontend Instructor // Code Syllabus

Common Pitfalls & Errors

The Error //

Assuming UV_THREADPOOL_SIZE takes effect if set after startup

// Correct: set before anything else runs
process.env.UV_THREADPOOL_SIZE = require('os').cpus().length.toString();
// ...rest of the app's requires/imports go after this line

The Solution //

libuv reads the thread pool size once at process startup — setting process.env.UV_THREADPOOL_SIZE after any asynchronous I/O has already occurred has no effect. Set it as the very first line of your entry file, or via the environment before launching node.

The Error //

Blocking the Event Loop with synchronous heavy computation

// Wrong: blocks the entire server
function heavySync() { for (let i = 0; i < 1e10; i++) {} }

// Correct: run on a worker thread
const { Worker } = require('worker_threads');
new Worker('./heavy-task.js');

The Solution //

A synchronous CPU-bound loop (like sorting a huge array or a naive Fibonacci) runs entirely on the single JS thread and blocks every other request until it finishes — no thread pool involved. Offload genuinely CPU-heavy JS logic to a worker_threads Worker instead of assuming it'll behave like the built-in async APIs.