Chaining Promises
Learn about the basics of the promise chain and find out the results of passing different arguments to the promise chain.
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One elegant feature of promises is that they form a pipeline like in the functional composition we discussed in the Arrow Functions and Functional Style lesson. Since both then()
and catch()
return a promise, calls to these functions may be chained to apply a series of filterings and transformations. An example will help you appreciate this elegance.
fs
library
In the example in No Thanks to Callback Hell, we used the fs
library’s readFile()
asynchronous method, which relied on callbacks. The fs-extra
library provides a wrapper around the functions of the fs
library. We will use this to read the contents of a file asynchronously, but this time, using promises instead of callbacks.
Wrapping Callbacks into Promises
fs-extra
is a library that wraps the callback-takingfs
library of functions intoPromise
-returning functions. This library works with only methods offs
, but what about your own legacy code or any third-party functions?The bluebird library was designed for this purpose. You can create a wrapper around any callback taking function using this library. Although you can design new asynchronous functions to work with
Promise
’s, use a library like bluebird to alleviate the pain of using legacy asynchronous functions.
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