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I'm learning Javascript with an online course. They say that the execution of Javascript is a "single threaded, synchronous execution". Then they define single thread as one mand at a time and synchronous as one at a time.
Is there a difference between these tho terms?
Do we monly say that Javascript is a synchronous language? Single threaded language?

I'm learning Javascript with an online course. They say that the execution of Javascript is a "single threaded, synchronous execution". Then they define single thread as one mand at a time and synchronous as one at a time.
Is there a difference between these tho terms?
Do we monly say that Javascript is a synchronous language? Single threaded language?

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Share Improve this question Follow asked May 29, 2017 at 10:34 user6827412user6827412 5

• 1 Yes there is a difference. And JS is not a synchronous lang. There are async tasks like timers, ajax etc – Rajesh Commented May 29, 2017 at 10:40
• @Rajesh Wrong. Answer – Suraj Jain Commented Jan 24, 2018 at 14:35
• @SurajJain can you explain? – Rajesh Commented Jan 24, 2018 at 14:43
• @Rajesh Sorry, Maybe you are right, I have to read much, someone who I know experienced programmer says javascript is syncronous in is behaviour. I am confused. – Suraj Jain Commented Jan 24, 2018 at 14:59
• @SurajJain js is synchronous but it's has features to add async tasks as well. Point is most of the code will be executed in top down manner but for some functions, engine will not wait for the task to finish. – Rajesh Commented Jan 24, 2018 at 17:43

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3 Answers 3

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Single threaded means that only one thing happens at a time.

Synchronous means that if you need to wait for something, then everything stops until the wait is over.

The most mon example of synchronous vs asynchronous in JavaScript is making an HTTP request.

If you make a synchronous request, then you send out the HTTP request over the network and then everything stops. Mouse clicks are ignored. Timers that reach zero at put on hold. Nothing happens until the response gets back.

If you want an asynchronous request, then the JS engine carries on with other work. When the request es back, and the JS engine isn't busy, then it is picked up and dealt with.

I found this and it really helped me to understand:

"In the end threading is about how many blocks of code (i.e. threads) we run on your puter's microprocessor simultaneously. If you have multiple 'cores' (like most modern Intel processors have) you can run multiple 'threads' simultaneously (i.e. each processor core is processing instructions at the same time).

Javascript engines don't do this. Javascript doesn't necessarily get faster with more processor cores.

On the other hand synchronous/asynchronous has to do with how a single thread is processed. Synchronous means 'wait for me to finish before doing something else'. Asynchronous means 'it's ok, keep going while I finish'. Javascript is synchronous and single-threaded. Only one thing is happening at a time within the engine, and only one set of instructions is being sent to your puter's microprocessor."

JavaScript as a Single-Threaded Language, which means it has one thread to execute all tasks. However, it employs the event loop mechanism to manage asynchronous operations, ensuring non-blocking behavior even with a single thread.

Synchronous Code:

Executes tasks one line at a time in a sequential, blocking manner. The JavaScript engine executes the code sequentially, particularly with operations that block further execution until pleted.

Examples: Voice calls, video calls (tasks that require blocking synchronous operations).

Asynchronous Code:

Allows multiple tasks to be initiated and executed without waiting for one to plete before starting another. Supports non-blocking behavior by leveraging the callback queue and event loop for scheduling operations and handling their pletion. Typically uses callbacks, promises, or async/await syntax to handle I/O operations.

Examples: Sending SMS, emails (non-blocking operations that proceed in the background).

In single-threaded programming, tasks are not necessarily executed synchronously. Asynchronous operations, aided by the event loop, enable JavaScript to handle non-blocking behavior effectively.

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