Image previews with the FileReader API
I added a “notes” section to this website eight years ago. I set it up so that notes could be syndicated to Twitter. Ever since then, that’s the only way I post to Twitter.
A few months later I added photos to my notes. Again, this would get syndicated to Twitter.
Something’s bothered me for a long time though. I initially thought that if I posted a photo, then the accompanying text would serve as a decription of the image. It could effectively act as the
alt text for the image, I thought. But in practice it didn’t work out that way. The text was often a commentary on the image, which isn’t the same as a description of the contents.
I needed a way to store
alt text for images. To make it more complicated, it was possible for one note to have multiple images. So even though a note was one line in my database, I somehow needed a separate string of text with the description of each image in a single note.
I eventually settled on using the file system instead of the database. The images themselves are stored in separate folders, so I figured I could have an accompanying
alt.txt file in each folder.
Take this note from yesterday as an example. Different sizes of the image are stored in the folder
/images/uploaded/19077. Here’s a small version of the image and here’s the original. In that same folder is the
This means I’m reading a file every time I need the
alt text instead of reading from a database, which probably isn’t the most performant way of doing it, but it seems to be working okay.
Here’s another example:
In order to add the
alt text to the image, I needed to update my posting interface. By default it’s a little
textarea, followed by a file upload
input, followed by a toggle (a checkbox under the hood) to choose whether or not to syndicate the note to Twitter.
The interface now updates automatically as soon as I use that
input type="file" to choose any images for the note. Using the
FileReader API, I show a preview of the selected images right after the file input.
Here’s the code if you ever need to do something similar. I’ve abstracted it somewhat in that gist—you should be able to drop it into any page that includes
input type="file" accept="image/*" and it will automatically generate the previews.
I was pleasantly surprised at how easy this was. The
FileReader API worked just as expected without any gotchas. I think I always assumed that this would be quite complex to do because once upon a time, it was quite complex (or impossible) to do. But now it’s wonderfully straightforward. Story of the web.
My own version of the script does a little bit more; it also generates another little
textarea right after each image preview, which is where I write the accompanying
I’ve also updated my server-side script that handles the syndication to Twitter. I’m using the
/media/metadata/create method to provide the
alt text. But for some reason it’s not working. I can’t figure out why. I’ll keep working on it.
In the meantime, if you’re looking at an image I’ve posted on Twitter and you’re judging me for its lack of
alt text, my apologies. But each tweet of mine includes a link back to the original note on this site and you will most definitely find the
alt text for the image there.