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javascriptasynchronous
larry burns
larry burns

Reputation: 189

Is javascript itself synchronous and it's the environment that is asynchronous?

Native javascript is single threaded and synchronous. Only a few objects can run asynchronously and get added to the callback queue such as HTTP requests, timers and events. These asynchronous objects are a result of the actual javascript environment and not javascript itself. setTimeout() seems to be the go to for examples for asynchronous code. That function gets moved to the web API container and then eventually the callback queue. There doesn't seem to be a way to write asynchronous code in javascript that doesn't involve using objects that get moved to the Web API container. I can write my own custom objects with callbacks, but the most that will do is structure it to run in the proper order. You can never write javascript code that runs in parallel that doesn't rely on those objects.

This is my understanding of how it works, if I am wrong please correct me.

Upvotes: 2

Views: 280

Answers (1)

Alex Wayne
Alex Wayne

Reputation: 187242

setTimeout and setInterval are just a convenient way to trigger some asynchronous behavior for an example. It is part of standard javascript library in all implementations, and not just part of the browser environment.

But all other sources of async code depend on some external process. When making an HTTP request, your javascript thread tells the browser to make a request (what headers, what url, etc). The browser, according to its own compiled internals, then formats the request, sends it, waits for a response, and eventually adds an item to javascript's event loop to be processed next time the event loop runs. File system access and database queries are two other common examples of async code that depends on external processes (the OS, and a database, respectively)

How javascript handles async code in a single threaded process is all down to this event loop. This event loop in psuedo code is basically this:

while (queue.waitForMessage()) {
  queue.processNextMessage();
}

setTimeout tells the environment to pop something into that queue at some point of the future. But the processing of that queue is single threaded. Only one event message can be handled at the same time, but any number can be added to that queue.

You can get true concurrency with workers, but this basically adds a new javascript process that is itself single threaded, and has a method of communicating with messages to and from the main javascript process.

Upvotes: 1

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