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

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

• Array.map() docs on MDN
• Axios repo on GitHub