PromiseFile

Overview

Sometimes we need to wait for an asynchronous call to return. Traditionally, JavaScript has handled this with callbacks. However, modern JavaScript uses Promises (and also async await) to handle this.

A Promise takes a function with two parameters:

• resolve: Function to use to return a value on success
• reject: A function to use to return a value on failure

Get Started

Be sure you first pull from JsObjects.

Create a folder called week08-promises and copy the code from this directory into it:

~/Git/JsObjects/JavaScript/EcmaScript6/PromiseSimple

Simplest

Consider this code which outputs the value 3:

const myPromise = new Promise((resolve, reject) => {
    resolve(3);
});

myPromise
  .then((result) => console.log(result));

This is the simplest possible promise that I can come up with. It differs from most promises in that it does not contain an asynchronous call. Note that:

• We create a promise with new
• It is passed by the runtime two functions called resolve and reject. In this case we don’t use reject but we do use resolve to return a value.
• We call the promise and use the runtime then function to print out the result sent by our promise.
  • Again, then is built into the JavaScript implementation of Promises. We don’t have to define it any more than we have to define or pass resolve and reject in the implmentation of our Promise.
  • Note that the value we resolve is passed as a parameter to then.

Moderate Difficulty

Consider this code which again contains no asynchronous call but which uses both resolve and reject:

const addNumbers = (a, b) => {
    return new Promise((resolve, reject) => {
        if (a && b) {
            resolve(a + b);
        } else {
            reject('Parameter missing');
        }
    });
};

addNumbers(2, 3)
    .then(result => console.log(result));

addNumbers(2)
    .then(result => console.log(result))
    .catch(error => console.log(error));

Here we:

• Define a function called addNumbers that takes two parameters that are to be added together.
• The function returns a Promise declared as in the previous example with resolve and reject.
• If the user passes in both parameters, we add them together and return them with resolve.
• Otherwise, we reject the call presumably because there are too few parameters. (I don’t cover the case where the user passes in something other than numbers.)

There are two calls to addNumbers.

• In the first we show what happens when we pass in valid input. The two parameters and added together and 5 is printed to the console.
• In the second we proide a catch function to handle errors. In this case it will print “Parameter missing” because we passed in only one parameter.

Write File

Here is a case where we use the NodeJs fs (File System) API to write the word foo to a file called foo.txt:

const fs = require('fs');

const elfWriteFile = (fileName, contents) => {
    return new Promise(function(resolve, reject) {
        fs.writeFile(fileName, contents, 'utf8', (err) => {
            if (err) {
                reject(err);
            }
            resolve({
                result: 'success'
            });
        });
    });
};

elfWriteFile('foo.txt', 'foo')
    .then(result => console.log(result));

Here we:

• Create a method called elfWriteFile that returns a promise
• elfWriteFile takes two parameters:
  • The name of a file
  • The contents to be written into the file
• Inside our promise we call the Node Js builtin fs.writeFile function.
  • When making the call we use both parameters passed to elfWriteFile
  • Note that we also specify the file type (utf8). We also do this when reading a file.
  • Note that the fourth parameter to writeFile, the one that begins with (err) is a callback function.
• If the JavaScript run time is unable to write the file to disk it returns an error and we reject the Promise.
• If the file is successfully written to disk we return a small object literal with a single key value pair set to result and success.

We call elfWriteFile with a file name and a small bit of text to be written to the file. You can, of course, pass in large strings in the second parameter.

Async Await

async function useAwait() {
    const foo = await elfWriteFile('bar.txt', 'bar async await');
    console.log('ASYNC AWAIT: ', foo);
}

useAwait();

Promisify

/* ==================================== */

const util = require('util');

const writeFile = util.promisify(fs.writeFile);
const readFile = util.promisify(fs.readFile);

async function callWriteFile() {
    const fileName = 'qux.txt';
    const encoding = 'utf8';
    try {
        await writeFile(fileName, 'quxor');
    } catch (error) {
        console.log(error);
    }
    const result = await readFile(fileName, encoding);
    console.log('READFILE RESULT', result);
}

callWriteFile();

Turn it in

Add a fourth Promise example. Wrap the Promise in a function called elfReadFile. It should use the asynchronous fs.readFile function to read the contents of foo.txt and it should then print it out. Don’t forget the utf8 parameter!

The docs for the Node Js fs module are here.

Convert String to Object

When you call js.readFile, the thing you get back is a string. But you want to use an object. In other words, because address-list.json contains JSON, we are tempted to expected the JSON in the file to be returned as an object, not a string. But that is not what you get. Regardless of the contents of the file, by default js.readFile returns a string. So you need to translate the string you get from js.readFile into a JavaScript object. Do it like this:

JSON.parse(resultFromCallingReadfile);

Here we call JSON.parse on the string returned by our call to fs.readFile.

NOTE: It is probably best that the thing resolved from readfile just be the string itself rather than object literal. Don’t do this: resolve({result: data}). Instead, do this resolve(data). Here data is the string passed to you in the callback of your Promise.

It is perhaps best to call JSON.parse just before, or while, you are calling response.send.

Here is a reference:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/parse