Synchronous:
A synchronous request blocks the client until operation completes. In such case, javascript engine of the browser is blocked.
Asynchronous
An asynchronous request doesn’t block the client i.e. browser is responsive. At that time, user can perform another operations also. In such case, javascript engine of the browser is not blocked.
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
