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Jeremy Keith
Jeremy Keith

Posted on • Originally published at adactio.com on

Image previews with the FileReader API

I added a β€œnotes” 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 alt text.

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 gistβ€”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 alt text.

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.

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