Javascript – Synchronous vs. Asyncronous

I’ve always been confused by the terms synchronous version asynchrous in Javascript, as they seem to mean just the opposite of what they would mean in the real world.  I always thought that synchronous meant multiple things could happen at once, and asynchrous means only one thing can happen at a time.

According to dictionary.com, I get the following meanings:

asynchronous:  not occurring at the same time.

synchronous: occurring at the same time; coinciding in time; contemporaneous; simultaneous.

From the MDN developer site, I get the following definitions:

asynchronous: Asynchronous refers to a communication environment where each party receives and processes messages when convenient or possible rather than immediately.

It can be used to describe a human communication environment like e-mail — the sender will send an e-mail, and the recipient will get round to replying to it when convenient; they don’t have to reply immediately.

It can also be used to describe a programatic communication environment, for example Ajax is an asynchronous mechanism for requesting small bits of data over HTTP; the result is sent back when the response is complete, not immediately.


synchronous:Synchronous refers to real-time communication where each party receives (and if necessary, processes and replies to) messages instantly (or as near to instantly as possible).

A human example is the telephone — during a telephone call you tend to respond to another person immediately.

Many programming commands are also synchronous — for example when you type in a calculation, the environment will return the result to you instantly, unless you program it not to.


When comparing here, I get a better understanding.  While Javascript is non-blocking (i.e. it doesn’t hold up execution while a piece of code runs), things are happening one at a time, just not necessarily sequentially.  According to the dictionary, sychronous means multiple things happening at once, while in Javascript it means something happens instantly (but not multiple things simultaneously).  

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