Guide to promises in Node.js

Written by Pascal Akunne

In Node.js applications, its not unusual to see a large number of nested callback functions being used to accomplish several activities. This is commonly referred to as callback hell, as it can make the code extremely complicated and disorganized.

Fortunately, theres a JavaScript solution called promises that solves the callback hell problem. This article will provide an overview of JavaScript promises and demonstrate how to use promises in Node.js with the promisfy() function.

In this article, well review the following:

• What is a promise?
• How do promises work?
• Creating a custom promise
• Consuming a promise
• Chaining promises
• Node.js promisfy() method

Prerequisites

In order to follow along, you should have the following:

• npm and Node.js installed
• Basic understanding of JavaScript
• VS Code installed, or the IDE of your choosing

What is a promise?

A promise is essentially an improvement of callbacks that manage all asynchronous data activities. A JavaScript promise represents an activity that will either be completed or declined. If the promise is fulfilled, it is resolved; otherwise, it is rejected. Promises, unlike typical callbacks, may be chained.

How do promises work?

JavaScript promises have three states: pending, resolved, and rejected.

The pending state is the initial state that occurs when a promise is called. While a promise is pending, the calling function continues to run until the promise is completed, returning whatever data was requested to the calling function.

When a promise is completed, it ends in either the resolved state or the rejected state. The resolved state indicates that the promise was successful and that the desired data is passed to the .then() method.

The rejected state indicates that a promise was denied, and the error is passed to the .catch() method.

Creating a custom promise

Promises are generally created by calling a Promise constructor, which accepts a single callback function as an argument. The callback function, also known as the executor function, is executed immediately after a promise is created.

The executor function accepts two callback functions as arguments, resolve and reject, which are referred to as function references. The resolve() and reject() functions each accept one argument, which could be a string, integer, Boolean, object, or array.

To better understand how to create a custom promise, lets look at the file, script.js:

function getSumNum(a, b) {  const customPromise = new Promise((resolve, reject) => {    const sum = a + b;    if(sum <= 5){      resolve("Let's go!!")    } else {      reject(new Error('Oops!.. Number must be less than 5'))    }  })  return customPromise}

Here, we define the function getSumNum() to compute the sum of two integers, a and b. Within the function, we use the promise constructor, new Promise(), to generate a new promise.

Next, we compute the sum of a and b. The resolve callback is executed if the sum is less than or equal to 5. Otherwise, the reject callback is called.

The new promise is passed to the customPromise variable, which is then returned. In the example above, we return a string, but it could also be an object or an array.

Now that we understand how a promise is created, lets review how it is consumed.

Consuming a promise

In application development, its much more common to consume promises than it is to create promises.

For example, when we request data from a server via an API that returns a promise, we utilize the then() and catch() methods to consume whatever data is delivered.

promise.then(data => {  console.log(data)}).catch(err => {  console.log(err)})

In the above code, the then() method is executed when the promise is fulfilled by the resolve() callback. The catch() callback is called if the promise fails, passing the error of reject().

Now, lets consume the promise we created previously:

function getSumNum(a, b) {  const customPromise = new Promise((resolve, reject) => {    const sum = a + b;    if(sum <= 5){      resolve("Let's go!!")    } else {      reject(new Error('Oops!.. Number must be less than 5'))    }  })  return customPromise}// consuming the promisegetSumNum(1, 3).then(data => {  console.log(data)}).catch(err => {  console.log(err)})

The sum of one and three is less than five, so the resolve() callback is run. This, in turn, executes the then() method. If we change the parameters to result in a sum greater than five, the reject() callback will be run and an error will be thrown using the catch() method.

Now, lets run the following command and then check the console:

node script.js

Chaining promises

Promises can be used to execute a series of asynchronous tasks in sequential order. Chaining multiple then()methods to a single Promise outcome helps avoid the need to code complicated nested functions (which can result in callback hell).

To demonstrate chaining promises, lets utilize the previous code with a few modifications:

let value;function getSumNum(a, b) {  const customPromise = new Promise((resolve, reject) => {    const sum = a + b;    if(sum < 5){      resolve(sum)    } else {      reject(new Error('Oops!.. Number must be less than 5'))    }  })  return customPromise}getSumNum(1, 3).then(data => {  console.log("initial data: " + data)  value = data + 1 // modifying the returned data  return value}).then(newData => {  console.log("modified data: " + newData)}).catch(err => {  console.log(err)})

Here, we see the result is passed through a chain of then() methods. We begin by declaring an empty variable called value. This time, instead of resolving a string, we pass the sum value.

When the initial promise object resolves, the then() function is invoked to log the initial data to the console before modifying the data by adding 1 and then assigning the resulting sum to the value variable. The value variable is passed to the next then() method, where the data is logged to the console.

Now, lets run the following command:

node script.js

Heres the output:

initial data: 4modified data: 5

Node.js promisfy() method

Promisification refers to a transformation. It is the conversion of a callback-accepting function into a promise-returning function. Promisification aids in dealing with callback-based APIs while maintaining code consistency.

Node.js has an inbuilt utility module, util.promisify(), that enables the creation of flexible promisification functions in JavaScript. util.promisify() takes a single function parameter, which contains the callback-based function.

Lets look at an example to better understand how to create a promisification function in Node.js.

First, we create two files, promisify.js and promise.txt.

In the promise.txt file, we add the following text:

Promisification refers to a transformation. It is the conversion of a callback-accepting function into a promise-returning function. Promisification aids in dealing with callback-based APIs while maintaining code consistency.

Next, we add the following code to the promisify.js file:

// Importing the fs moduleconst fs = require('fs');// Importing util moduleconst util = require('util');// Use promisify to fs.readFile to promise based methodconst readFile = util.promisify(fs.readFile);readFile('./promise.txt', 'utf8') // Reading the .txt file.then((text) => {console.log(text);})// Log error if any.catch((err) => {console.log('Error', err);});

To read the files in the above example, we utilize the fs module. Then, we use the util.promisify() technique to transform the fs.readFile into a promise-based function. Instead of a callback, the above method now returns a promise.

Now, lets run the following command: node promisify.js

We see that the text from the promise.txt file is logged to the console:

Conclusion

When developing Node.js applications, its important to understand how to make optimal use of promises. Compared to the usual callback function, promises provide a clearer, more flexible, and better organized manner of managing asynchronous operations.

In Node.js, we can use the util.promisify() utility module to easily transform a standard function that receives a callback into a function that returns a promise.

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