Answer
Synchronous blocks the main thread; asynchronous uses callbacks/promises and does not block.
Quick Answer
Synchronous XHR blocks the main thread until the server responds — freezes the page. Asynchronous XHR/fetch is non-blocking — the code continues and a callback/Promise fires when the response arrives. Never use synchronous XHR in production. fetch() is the modern, Promise-based replacement for XHR — cleaner API and supports async/await.
S
SugharaIQ Editorial Team
Verified Answer
This answer has been peer-reviewed by industry experts holding senior engineering roles to ensure technical accuracy and relevance for modern interview standards.
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Source: SugharaIQ