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javascriptnode.jsasynchronous
Jake Wilson
Jake Wilson

Reputation: 91193

What makes a JavaScript function asynchronous?

I am aiming to understand what exactly is it about certain JavaScript functions that make them asynchronous. You could have a function like this:

function someRandomFunction () {
  for (let i of everything) {
    iterator++
    something = true
    hello = that + whatever
  }
}

Nothing about this function is asynchronous. It does a lot of stuff but it does it very quickly.

But then take a Node.js function like this:

fs.readFile('path/to/file', (err, data) => {
  // do something
})

This function is declared to be asynchronous. But why is it? What is the reason behind it?

Is it because it takes a certain amount of time for reading a file to complete, therefore it's asynchronous? Why is that asynchronous when looping through some variables and doing some calculations is not asynchronous?

Upvotes: 2

Views: 442

Answers (4)

Redu
Redu

Reputation: 26161

This is a basic but fundamental question. Passing a function to another function doesn't make the code asynchronous automatically. It's all about attemting to work with another piece of code, mostly of unknown origins, probably but not necessarily related with operating system's IO mechanism. In another words you ask something to happen out of the realm of the currently exectuing JS context by handing a task to known or unknown resources which are possibly running on a separate thread.

As i said this can be an IO operation or a database access of which you have no idea on

  • What code is running
  • At which thread it is running
  • How long it will take
  • Whether it will succeed or not

Behind the curtains these alien codes finally trigger the event queue to invoke the callback function through a driver. In some pure functional languages like Haskell such activities are strictly kept away from the context and must be handled by monadic type functions. In JS this is basically done by asynchronous workflow by callbacks, promises or async-awiat mechanism.

Once you hand out a task to the outer world then there shall be no way for it to interfere with your synchronous code.

Upvotes: 2

Vinnie James
Vinnie James

Reputation: 6052

In terms of JavaScript, the difference between synchronous and async functions is where and when the code is executed. Synchronous functions are executed immediately one after the other in the current call stack. Async functions are passed off to the event loop, and values returned once the current call stack has completed execution.

Caveat being, nothing in JavaScript is truly asynchronous due to the fact that JS only executes on a single process thread

Upvotes: 1

David
David

Reputation: 197

A function is either explicitly synchronous or asynchronous; there's no "certain amount of time" that automatically makes it asynchronous.

The important thing to remember with Node is that it's "async I/O", which means nothing is ever async without some input/output to/from the process. You normally see this with file reading/writing, database calls, and anything that requires a network call.

Upvotes: 2

Randy Casburn
Randy Casburn

Reputation: 14165

The 'idea' of something being asynchronous means "we've relinquished control to some other operational code, and we have no idea when that other code will allow us to operate again".

So in your bottom sample, you give control over to node's filesystem operations to read a file. We don't know how large that file is, so there is no way to anticipate how much time it will take to complete. So JavaScript allows us to provide a "callback" that get's fired when the async operation is complete. When the callback is executed, control is returned to our code.

The top example is synchronous because your code maintains control over the operation and Javascript executes code synchronously by nature.

Upvotes: 5

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