When to use 'async' built-in for ES2017 or import 'npm i async'-CodePudding

I recently learned that async/await is built into ES2017, but I have made several projects that I needed to require the package async in order to use async/await.

Is there an easy way to tell when I can access async normally or when I need to import it? Do I ever need to use that npm package? What is the purpose of the async package (which currently shows 47,469,002 weekly downloads) if the exact same functionality is now built into the language?

For an example project that requires async feel free to look at the Local-Library MongoDB/Express/Node tutorial project on MDN.

Since this is an Express app (as several of my own are), does this have anything to do with ExpressJS?

I have looked around for about a bit trying to find these answers but have come up empty-handed. Any help understanding this is appreciated.

CodePudding user response：

The async library on NPM provides a number of utility functions for managing asynchronous operations. That is very different than the async keyword in the language that allows you to use await with promises. These are like cats and dogs. They both have something to do with asynchronous programming, but other than that, they really aren't the same thing at all.

For example, suppose you need to make 1000 separate requests to a particular host, but for a variety of reasons (memory consumption, rate limiting by the host, etc...), you cannot have more than 5 requests to target host in flight at any given time. So, you want to launch 5 requests and then each time one finishes, you'll launch another one until you've finally done all 1000.

The async keyword in ES7 could perhaps be used in some custom code to implement the algorithm above I described, but by itself, it doesn't give you that solution.

Whereas the async library from NPM has a specific function in its toolkit for doing exactly what I described. It's called parallelLimit() and it lets you specify exactly how many operations you want to be in parallel at a time (with no more than that). In fact, the async library contains all sorts of utility functions for managing asynchronous control flow and you can see a whole list of them here: https://caolan.github.io/async/v3/docs.html#parallelLimit.

Now that we have the async keyword in the language, some of those algorithms are no longer needed because it's very easy to code them in plain ES7. For example if you want to iterate over an array one at a time, calling an asynchronous function on each item, you can just use a for loop and await, all inside an async function.

async function run() {
    for (let x of myArray) {
         await doSomeAsyncOperation(x);
    }
}

Before we had async and await in ES7, you would have had to either write a bit of custom code to do this asynchronous, serialized iteration or you would use a pre-built function from a library such as the async library.

Summary

To review, the async library contains a number of utility functions for managing asynchronous operations. Some of those are no longer necessary because of the control flow options that async and await provide in ES7 and some are still helpful/useful even with the presence of async and await.

FYI, the async library was originally developed before we even had promises and async/await in Javascript and it dealt with the older-style asynchronous operations that used plain callbacks to signal completion or error and offered utilities for managing them. Promises and async/await have replaced the need for some of the functionality in the async library, but not all.

When to use 'async' built-in for ES2017 or import 'npm i async'

Use the built-in ES7 async/await when it directly and simply solves the asynchronous control flow problem you need to solve. Use a function from the async library when you can't easily solve your problem with just async/await and the async library contains a pre-built function that solves your problem directly and simply.

So, the async library is just one of hundreds of thousands of libraries on NPM. You use it when it contains something that helps you solve your job better than what is already built-into the language, the same logic for when to use any other module from NPM.

CodePudding user response：

aysnc/await that are part of ES2017 enables cleaner style there by avoiding promise chains. See the below example mentioned in mdn. You avoid promise chains like then, catch and yet have asynchronous behaviour https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function

Async library that you mentioned is more of a utility( they mainly designed to use with node js), It takes the array of callbacks, executes and wraps the them and gives you a promise. Iy also enables you with lot more features like in case you want invoking callback in parallel, serial, chain the callbacks, there by helping track these with a common success/error handling