What is the difference between synchronous and asynchronous JavaScript?

In synchronous JavaScript, operations are executed one after another, meaning each operation must complete before the next one begins. This can lead to blocking if an operation takes a long time to complete. Asynchronous JavaScript allows for operations to run in the background, freeing up the main thread to continue executing other code. Callbacks, Promises, and async/await are common ways to handle asynchronous operations. 

// Synchronous example
console.log('Start');
for (let i = 0; i < 1000000000; i++) {} // Blocking loop
console.log('End');

// Asynchronous example
console.log('Start');
setTimeout(() => console.log('Async operation complete'), 1000);
console.log('End');