What is the difference between synchronous and asynchronous JavaScript?

Direct answer

Synchronous code completes in sequence on the current call stack, while asynchronous workflows allow work to finish later and schedule continuation code without blocking the entire program for that operation.

JavaScript execution on a given main thread runs one piece of JavaScript at a time, but host environments provide asynchronous capabilities such as timers, network requests, and events.

Synchronous: the next statement waits until the current JavaScript work finishes.
Asynchronous: an operation can be initiated now and its continuation handled later through callbacks, promises, events, or async/await.