2. Tutorials
  3. Node.js — How to Run an Asynchronous Function in Array.map()

Node.js — How to Run an Asynchronous Function in Array.map()

Array.map() is a synchronous operation and runs a function on each element in the array resulting in a new array with the updated items.

There are situations where you want to run asynchronous functions within map, e.g. updating a list of models and push the changed information back to the database or request information from an API that you want to use for further operations.

Let’s solve the problem of running an asynchronous operation in a synchronous function!

Node.js Series Overview

  1. Increase the Memory Limit for Your Process
  2. Why You Should Add “node” in Your Travis Config
  3. Create a PDF from HTML with Puppeteer and Handlebars
  4. Create Your Own Custom Error
  5. Retrieve a Request’s IP Address in Node.js
  6. Detect the Node.js Version in a Running Process or App
  1. String Replace All Appearances
  2. Remove All Whitespace From a String in JavaScript
  3. Generate a Random String in Node.js or JavaScript
  4. Remove Extra Spaces From a String in JavaScript or Node.js
  5. Remove Numbers From a String in JavaScript or Node.js
  6. Get the Part Before a Character in a String in JavaScript or Node.js
  7. Get the Part After a Character in a String in JavaScript or Node.js
  8. How to Check if a Value is a String in JavaScript or Node.js
  9. Check If a String Includes All Strings in JavaScript/Node.js/TypeScript
  10. Check if a Value is a String in JavaScript and Node.js
  11. Limit and Truncate a String to a Given Length in JavaScript and Node.js
  12. Split a String into a List of Characters in JavaScript and Node.js
  13. How to Generage a UUID in Node.js
  1. Filter Data in Streams
  1. Get Number of Seconds Since Epoch in JavaScript
  2. Get Tomorrow’s Date in JavaScript
  3. Increase a Date in JavaScript by One Week
  4. Add Seconds to a Date in Node.js and JavaScript
  5. Add Month(s) to a Date in JavaScript or Node.js
  6. Add Week(s) to a Date in JavaScript or Node.js
  1. How to Run an Asynchronous Function in Array.map()
  2. How to Reset and Empty an Array
  3. for…of vs. for…in Loops
  4. Clone/Copy an Array in JavaScript and Node.js
  5. Get an Array With Unique Values (Delete Duplicates)
  6. Sort an Array of Integers in JavaScript and Node.js
  7. Sort a Boolean Array in JavaScript, TypeScript, or Node.js
  8. Check If an Array Contains a Given Value in JavaScript or Node.js
  9. Add an Item to the Beginning of an Array in JavaScript or Node.js
  10. Append an Item at the End of an Array in JavaScript or Node.js
  11. How to Exit and Stop a for Loop in JavaScript and Node.js
  12. Split an Array Into Smaller Array Chunks in JavaScript and Node.js
  13. How to Get an Index in a for…of Loop in JavaScript and Node.js
  14. How to Exit, Stop, or Break an Array#forEach Loop in JavaScript or Node.js
  15. Retrieve a Random Item From an Array in JavaScript or Node.js
  1. Callback and Promise Support in your Node.js Modules
  2. Run Async Functions/Promises in Sequence
  3. Run Async Functions/Promises in Parallel
  4. Run Async Functions in Batches
  5. How to Fix “Promise resolver undefined is not a function” in Node.js or JavaScript
  6. Detect if Value Is a Promise in Node.js and JavaScript
  1. Human-Readable JSON.stringify() With Spaces and Line Breaks
  2. Write a JSON Object to a File
  3. Create a Custom “toJSON” Function in Node.js and JavaScript
  1. Check If a Value Is Iterable in JavaScript or Node.js
  1. Extend Multiple Classes (Multi Inheritance)
  2. Retrieve the Class Name at Runtime in JavaScript and Node.js
  1. Generate a Random Number in Range With JavaScript/Node.js
  2. Ensure a Positive Number in JavaScript or Node.js
  1. How to Merge Objects
  2. How to Check if an Object is Empty in JavaScript or Node.js
  3. How to CamelCase Keys of an Object in JavaScript or Node.js
  4. How to Snake_Case Keys of an Object in JavaScript or Node.js
  5. How to Destructure a Dynamic Key in JavaScript or Node.js
  6. How to Get All Keys (Including Symbols) from an Object in JavaScript or Node.js
  1. Get a File’s Created Date
  2. Get a File’s Last Modified/Updated Date
  3. How to Create an Empty File
  4. Check If a Path or File Exists
  5. How to Rename a File
  6. Check If a Path Is a Directory
  7. Check If a Path Is a File
  8. Retrieve the Path to the User’s Home Directory
  9. How to Touch a File
  10. Read File Content as String
  11. Check If a Directory Is Empty
  12. How to Create a Directory (and Parents If Needed)
  1. Create From Object
  2. Transform to an Object
  1. Determine the Node.js Version Running Your Script

Use Promises to Run Async Operations in Array.map

Iterating through an array in JavaScript with .map() expects a synchronous operation and a return of an (updated) value. Well, you could do a trick and return a promise. The returned promise is a value that satisfies map and ultimately represents an async operation 🤘

Return a Promise for Each Array Item

An approach to perform async actions in array.map() is to return a promise for each item which then resolve outside the map function. Because map won’t wait for the promise to resolve, it’ll return a pending promise.

If you’d console log the resulting array from map, it looks like this:

[
  Promise { <pending> },
  Promise { <pending> },
  …
]

You need to take care of all promises in the array returned from map to resolve before using their results. Do this with Promise.all(<array-of-promises>). The result of waiting for all promises to finish is another array containing the results.

Let’s visualize the idea with an example in the upcoming paragraph.

Complete Example: Fetch Repos From GitHub API

Let’s say you’ve an array of GitHub repository URLs and you want to fetch the repo’s name and description for more context.

The sample code below iterates through the array of repository URLs and fetches the repository data from GitHub’s API using Axios. Notice that the wrapping function fetchRepoInfos is an async function. The async and await operators are available in Node.js v8. Use plain promises if you’re with Node.js v4 or v6.

Returning a value from an async function automatically results in a promise. You don’t necessarily need to use an async function. Returning a created promise, like from Promise.resolve(<data>), does the same thing.

async function fetchRepoInfos () {  
  // load repository details for this array of repo URLs
  const repos = [
    {
      url: 'https://api.github.com/repos/fs-opensource/futureflix-starter-kit'
    },
    {
      url: 'https://api.github.com/repos/fs-opensource/android-tutorials-glide'
    }
  ]

  // map through the repo list
  const promises = repos.map(async repo => {
    // request details from GitHub’s API with Axios
    const response = await Axios({
      method: 'GET',
      url: repo.url,
      headers: {
        Accept: 'application/vnd.github.v3+json'
      }
    })

    return {
      name: response.data.full_name,
      description: response.data.description
    }
  })

  // wait until all promises resolve
  const results = await Promise.all(promises)

  // use the results
}

Outside the map function, await all promises to resolve. If you’re not on the async/await train yet, chain .then(results => { … }) call on Promise.all() to wait for everything to finish.

If you’d console log the results, it’ll will give you an array of data like this:

[
  {
    name: 'fs-opensource/futureflix-starter-kit',
    description: 'Starter kit for the “learnhapi.com” learning path'
  },
  {
    name: 'fs-opensource/android-tutorials-glide',
    description: 'Example code for Glide tutorial series'
  }
]

The result will be another array containing the data returned from map and resolved from promises. Starting here, you’d do further processing or operations on the results.

Summary

Array.map in Node.js is a powerful feature to iterate through a list of values and manipulate each item. The downside of map is that you can need additional steps to run asynchronous tasks within the synchronous map function.

Promises are a solution to solve the synchronous aspect. Run asynchronous tasks within map by returning a promise and waiting for the new array of pending promises to resolve. Ultimately, operate on the resulting array with actual values.

We love to hear your thoughts and ideas. Tell us if promises fit your needs to run asynchronous operations within Node.js’ .map(fn) function. Use the comments below or tweet us @futurestud_io.

Mentioned Resources

Marcus Pöhls

Marcus is a fullstack JS developer. He’s passionate about the hapi framework for Node.js and loves to build web apps and APIs. Creator of Futureflix and the “learn hapi” learning path.

Explore the Library

Find interesting tutorials and solutions for your problems.

Android

Node.js

Server

Miscellaneous