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Goodbye Transform-Streams, long live ES9 Async Generators

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[前端(javascript) 所属分类 前端(javascript) | 发布者 店小二03 | 时间 2018 | 作者 红领巾 ] 0人收藏点击收藏

Goodbye Transform-Streams, long live ES9 Async Generators
TL;DR.

Async generators are better than transform streams. This is not a tutorial.

What are transform streams?

Node.js doc says: “a transform stream is a duplex stream where the output is computed in some way from the input. Examples include zlib streams or crypto streams that compress, encrypt, or decrypt data”.

I’m not going to explain them here, you can find lot of free resources online. They are a really powerful resource to build amazing things with Node.js, but the drawback is that they are not so simple to implement not even for easy stuff.

How does async generators come intoplay?

Async generators are a special ES9 news that let us to easily implement the asynchronous iterator interface. Node.js streams have already implemented it, allowing us to do like the following:

So if a stream is readable it is also an asynchronous iterable.

This is not a tutorial on async stuff, if you want to know more about async iterables and async generators check this out !

In short words async generators let you:

transform chunks pipe chunks’ transformations filter/compose chunks before passing them to the next async generator/writing them to the output stream.

Let’s see an example where we pipe two generator for filtering out all even numbers and increment by one odd ones, before writing them to process.stdout . Numbers are asynchronously coming from a misterious readable stream:

Are not you confident with generators?

Don’t worry. I’m working on a little module that let you write functions, also async ones, and then trasform them into async generators! Furthermore I’ve coded a little helper function to easily pipe async generators together.

Let’s see how to use the asyncGeneratorsFactory :

You can see that filter functions must return undefined only when a chunk has to be discarded. Normal transform functions must always return a value. Composition functions are not yet supported, but I’m working on them.

Now let’s see how to pipe the generators:

It’s great, isn’t it?

Here you can find a complete gits.

Here you can see the source code of my helpers.

What about performance?

I’ve tested async generators vs transform streams with files. With small and medium size files you cannot see lot of differences: sometimes transform streams are better, sometimeas async generators are better.

If you need to use my asyncGeneratorsFactory helper function know that things will go a bit slower.

Anyway with big size files seems that async generators are better. But more tests are needed also with different types of streams. If you can, help me:)

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