Ryan Cao
Projects Uses Blog

Building a Unsplash Downloader with Deno

~4 min


This article may contain outdated information, as it is more than two years old.

A few days ago, a friend asked me to help him download 25,000 images from Unsplash as 250x250 thumbnails in order to feed it into his machine learning model. This was an interesting task, and I decided to write it in Deno.

If you haven’t heard of Deno, it’s a runtime for JavaScript, similar to Node, that has more features such as built-in TypeScript, ESM imports, a security sandbox, and many others. The ecosystem hasn’t quite fully developed yet for more production uses, but it’s quite nifty for writing little scripts like renaming files programmatically, etc.

So I set out to write this script.

$ mkdir unsplash-dl && cd $_

Input #

My friend provided me with a data.json that contained a list of all the Unsplash image IDs, like this:

  // ...

So first of all in the script, we have to read the JSON file and parse it.

const imgList: string[] = JSON.parse(await Deno.readTextFile("./data.json"));

We define imgList’s type to be string[] explicitly for better type inferences, since by default JSON.parse returns an object with type any.

And now we have the list of image IDs.

Download #

First, we have to get the URL for the actual Unsplash image, resized to 250x250. Fortunately, Unsplash has a service source.unsplash.com which provides these image resizing features with a simple URL scheme: https://source.unsplash.com/{id}/{width}x{height}.

It would be better to put this functionality into a function by itself, so:

const download = async (img: string) => {
  const url = `https://source.unsplash.com/${img}/250x250`;

  // ...

Then, we have to actually download the image.

Note: These images were verified to be JPEG images, so we can save them as .jpg files directly.

This required the writeAll function, so add this to the top of the script:

import { writeAll } from "https://deno.land/[email protected]/io/util.ts";
const download = async (img: string) => {
  const url = `https://source.unsplash.com/${img}/250x250`;
  const res = await fetch(url);

  if (!res.ok || !res.body) {
    const percentage = ((count / 25000) * 100).toFixed(2);
      `[${percentage}%] ${img} download error: ${res.status} ${res.statusText}`,


  const file = await Deno.open(`./downloads/${img}.jpg`, {
    create: true,
    write: true,

  for await (const chunk of res.body) {
    await writeAll(file, chunk);


We log the error if the response is empty or has a non-2xx status code and continue. If the response is okay, we split the body into chunks and write them to a file in the ./downloads/ directory.

Do note that you have to create the downloads/ directory manually, or else it would throw an error.

For good measure, let’s add a counter to see where we are in our progress.

// Add this before the `imgList` definition

let cnt = 0;
// Add this after `file.close();`

const percentage = ((count / 25000) * 100).toFixed(2);
console.log(`[${percentage}%] ${img} downloaded`);


We display a percentage with two places after the decimal to show our progress.

And that is our download function.

Finding undownloaded files #

There is always the possibility of Internet outages or accidental shutdowns or some other unfortunate accident which interrupts our script, so we have to be prepared to resume / pause the script at any time.

Fortunately, we can detect whether the downloaded file exists to see if any image has been downloaded. Deno has no built-in function to detect whether a file exists, but if you run Deno.stat on a nonexistent file it will throw an error, so we can implement our exists function like this:

const exists = async (imgId: string) => {
  try {
    await Deno.stat(`./downloads/${imgId}.jpg`);
  } catch {
    return false;

  return true;

Then, we filter our imgList for only the ones that actually need downloading, so edit your original definition of imgList to add a filter. Array.prototype.filter doesn’t support using async functions yet, however, our exists function is an async function, so we’ll have to use this workaround:

const imgList: string[] = JSON.parse(await Deno.readTextFile("./data.json"));
let filteredImgList: string[] = [];

for (const img of imgList) {
  if (await exists(img)) {
  } else {
    filteredImgList = [...filteredImgList, img];

Download the list with concurrency #

We would like to download these images with concurrency, since that would be faster; however, we wouldn’t want infinite concurrency like Promise.all’s default behavior, because that would make each request very very slow and thousands of simultaneous writes to the filesystem. Which does not bring optimal performance.

We can limit concurrency via the p-limit library. We import it at the top of our script, from Skypack:

import pLimit from "https://cdn.skypack.dev/p-limit?dts";

?dts adds type information for Deno.

Then, add this after your definition of the download function:

const limit = pLimit(5);

await Promise.all(
  filteredImgList.map((i) => {
    return limit(() => download(i));

The limit function takes in our async function and returns an async function that has concurrency limited to 5, which (from trial and error) seems to be most optimal for download speed.

Run the script #

If you’ve lost track of what we’ve written so far and where to put the code blocks, here’s the full script.

Now that our script’s finished, let’s run it in the terminal!

$ deno run -A mod.ts

[0.00%] XMyPniM9LF0 download successful
[99.97%] 0fh58lD8AuI download successful
[99.98%] 0s0WCiys0ZI download successful
[99.98%] 0SqUthjger8 download successful
[99.98%] 0HGKG23yMew download successful
[99.99%] 0FVQQhggj2o download successful
[99.99%] 0ex5ixoTnRw download successful
[100.00%] 0PHJAnuCDtA download successful

It works beautifully. 🥳🥳🥳

That’s all for this post! I hope you had fun building this little Deno script 😊

Licensed under CC BY-SA 4.0

deno javascript typescript scripting

Published on 2021-08-10

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