CODEWITHSUNDEEP
javascriptnode.jspromisechaining
Flame_Phoenix
Flame_Phoenix

Reputation: 17574

Accomplish Promise Chaining

Background

I have a nodejs server running and I installed the promise package which follows the promise api specs.

Since I succeeded in making denodeify(fn, length) work, I am now in the process of chaining promises, but I am failing to grasp the main concepts.

What I tried

By reading the documentation example on the specification page, I reached the following code:

let Promise = require("promise");
let write = Promise.denodeify(jsonfile.writeFile);
let read = Promise.denodeify(jsonfile.readFile);

read("dataFile.txt").then( () => {
    write("./testFile.txt", "hello_World", TABS_FORMATTING).then(console.log("file complete"));
});

Which is quite different from the examples I see, for example, in the Solutions Optimist tutorial:

 loadDeparture( user )
        .then( loadFlight )
        .then( loadForecast );

Objective

My objective is to make my code as beautiful as the example I showed before, but I don't understand how I can make as concise as it is right now.

Question

1 - What changes do I need to perform in my code to achieve that level?

Upvotes: 0

Views: 71

Answers (2)

grimurd
grimurd

Reputation: 2850

The given example uses named function to make it look as good as it can get, but that can be a bit redundant because then you're creating functions for every little thing in the chain. You must pick and choose when to use named functions over anonymous functions.

One thing you must also realize is that to chain promises you must return them. So to make it a proper chain you must return the write method so it is passed down to the next step.

Also make sure that the catch() method is used at the bottom of every promise chain so that errors aren't silently swallowed.

Note that in the example here I'm using the ES2015 arrow functions to return the write() method as that makes it looks better(which seemed to be the purpose of your question).

let Promise = require("promise");
let write = Promise.denodeify(jsonfile.writeFile);
let read = Promise.denodeify(jsonfile.readFile);

read("dataFile.txt")
    .then(() => write("./testFile.txt", "hello_World", TABS_FORMATTING))
    .then(results => console.log("file complete", results))
    .catch(error => console.error(err));

I'd recommend reading this article for some best practices.

Upvotes: 1

Davin Tryon
Davin Tryon

Reputation: 67296

Nesting promises kind of defeats the purpose because it creates pyramid code (just like callbacks).

The main concept that may be escaping you is that you can return inside a then and the returned value (can be a promise or a value) can then be accessed in a chained then:

read("dataFile.txt").then( () => {
    return write("./testFile.txt", "hello_World", TABS_FORMATTING);
}).then( () => {
    console.log("file complete");
});

Now, you can extract the functions:

function writeTheFile() {
    return write("./testFile.txt", "hello_World", TABS_FORMATTING);
}

function consoleLog() {
    console.log("file complete");
}

read("dataFile.txt")
    .then(writeTheFile)
    .then(consoleLog);

Upvotes: 1

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