The answer
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.
Synchronous Execution in JavaScript Explained
Is fetch synchronous or asynchronous in JavaScript?
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.
All other work stops. Click handlers, timers, and animations cannot run until the blocking function returns. The UI freezes for the duration.
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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