What happens when a function blocks the main thread in JS?
All other work stops. Click handlers, timers, and animations cannot run until the blocking function returns. The UI freezes for the duration.
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More FAQs in Synchronous Execution in JavaScript Explained
Each statement runs one at a time, in order, on a single call stack. The next line waits for the previous line to finish before it can run.
Asynchronous. fetch returns a Promise immediately. The network call runs in a Web API, and the .then callback runs on the call stack only when the stack is empty.
Move heavy computation to Web Workers, break large loops into chunks with setTimeout or requestIdleCallback, and prefer async APIs like fetch and FileReader over synchronous equivalents.
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