Better Image Previews in HTML5

When working with an uploader it’s nice to show image previews of files before the actual upload takes place. It looks nice and verifies to the user that the correct files have been selected from their computer. Since they are local files people expect them to render immediately. This article details how I accomplished this in HTML5 without the use of flash, silverlight or any other dependent technology.

Over the last several weeks I’ve been researching Plupload for image uploads. They have a nice, ready-made widget but it didn’t quite capture the look and feel that was requested of me. I opted to rely on their API and do the interfacing myself, which included developing my own image previews.

My main requirement was for them to render quick, even with 20MB files. My project was tailored to professional photographers so big files were expected. Searching the web I tried a few techniques that seemed to perform well with small files but would max out resources for larger ones. Things became choppy. A page quickly looks cheap when buttons don’t respond to clicks, content freezes intermediately, etc. I had to come up with a different solution. Along with speed, I also wanted to keep consistent thumbnail sizes and perform ideal cropping. Image quality had to be fair and recognizable as well.

The first function handles proportional resizing. I needed to be able to retrieve lower dimensions that didn’t skew or distort an image:
In most cases you’ll end up with one property that meets the max, but the other dimension will be much smaller than its max. For thumbnails to look clean you need a consistent width and height for every image. So that means you will likely need to crop. But you don’t necessarily want to perform that action on the entire image. Everything becomes so small it becomes hard to read. Rather grab a portion of the image as your scaling down. The following function utilizes proportionalResize as it determines the best coordinates for presenting your image:
Finally, we get to the meat of the process. This function returns the preview as a <canvas> element for your page. A <canvas> tag resembles an <img> tag with one big difference. Instead of loading the image from the network it’s drawn via JavaScript. It also has a built-in cropping mechanism. If using Plupload pass the result of file.getNative(). If not refer to this resource for getting a FileList of File objects to be passed in individually. This function calls the aforementioned centerCropFit function:
Originally, I tried Filereader.readAsDataURL() which converts the entire file into base64-encoded string. Whenever I assigned it to the src of an image it would cause a big memory hit for large files. Attempts to use window.setTimeout() to get around the heavy processing did not help. I also explored Web Workers to no avail. The URL.createObjectURL() method is the nitrous behind this process and allowed the tool to perform well for the user.

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Ensuring Variable Integrity in Async Callback Functions with JavaScript Closures.

Callback functions to asynchronous requests may need to reference variables outside of itself. This can be problematic because those variables might not maintain the values you expect by the time the callback function fires. In the past I’ve dealt with this by writing my code as a string, concatenating my variable values, then using eval on it in the callback. I never liked this technique because it seemed amateur, risky and unreliable for complex scenarios. Recently I came across a clever solution that seems to work quite well:
Here we wrap a function around the code, and execute it immediately passing the target variables into it. In my example the variables are file and uploader. Next, we then have an inner function that keeps the meat of our code from being executed immediately. It will be executed in the callback of the asynchronous process…but can reference the variables passed to it’s wrapper function. In the example I have used the same variable names again (no need to change them). I believe the technique duplicates the variables into a local scope where its values can be maintained. This is also known as a JavaScript Closure where functions have access to private variables that nothing else can touch or modify.

Here is a FIDDLE showcasing the technique. Enjoy.

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Creating Animated Spinners with FontAwesome or Glyphicons

When creating JavaScript-rich applications an indicator is often used to give the user some certainty the page is working behind the scenes. If not present a user may question if the page has stalled or not. An animated GIF known as a spinner (or loader) is a common way to achieve this. An alternative to linking to a specific image is to use a popular font library that you may already be referencing by your application.


The ability to create spinning icons in built into FontAwesome. Reference the library and create loaders easily:
The fa-spin class is what causes the rotation.

If you desire a different animation speed then you need a little CSS:
FYI: this CSS was constructed by studying the fa-spin class found in the FontAwesome stylesheet. Adjust the milliseconds (i.e., 1000ms) to alter the speed. Higher is slower, lower is faster. The default speed used by FontAwesome is 2000ms.

To use simply replace the fa-spin class with fa-spin-custom class that you have made:


To use the font-library built into Bootstrap you will need to reference the above CSS as well:

That’s it. See a DEMO.

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