The Flexbox module for css was built with the intent to make a more robust, less hacky way to layout elements on pages. When you’re building a webpage you often don’t know how high or wide every element could or should be. This can cause problems in certain layouts which lead to ugly hacks. Flexbox solves this widespread layout issue. The module has been in development for quite some time and the W3C gave the spec a “last call working draft” status back in september of 2014. The browser support for this module is very good if you don’t have to support IE9 so you can safely use it. In this post I will provide a few code examples to show you how you can use Flexbox in some everyday situations.

Creating a filmstrip

Have you ever created a horizontally scrolling filmstrip kind of module? You know the size of the each element in the strip and the size of the containing element but the size of the inner element, the filmstrip itself is unknown. This would normally result in a layout like this and you would have to use Javascript or hardcode the size of the inner element to make this work.

So what would happen if you used Flexbox for this? Well, Flexbox allows an element to grow, not only on the y axis like an element normally does, but also on the x axis. It’s one of the reasons Flexbox is really cool. Let’s try this.

It’s pretty neat, isn’t it? Between the first and second example there’s only three lines of code that are different. Okay, actually there’s a little more but I’m not counting in the vendor prefixes because you don’t have to write those if you use an autoprefixer. The first difference is that we give  overflow-x: scroll; to the filmstrip container, that’s just to make the contents scroll. The second step is to set  display: flex; on the inner element. If you did only these two things, the items inside of the inner element will shrink to fit inside of their container. You don’t want this so the last thing you do is add  flex-shrink: 0; to the filmstip items. The shrink property has a value of 0(false) or 1(true). There’s also a  flex-grow  property, it’s the same as the shrink property but it determines whether an element will grow or not.

Vertical centering

Ever since I started writing css this has been a problem. How do you center an element, with or without a known height, in a container that is or isn’t flexible? No matter how you look at it, vertical centering is annoying. I’ve used hacks that would absolutely position the centered element at 50% from the top and then I would use a negative top margin to push the element towards the center. Another method is to use translating which is slightly cleaner but you still have to use position absolute and a top offset of 50%. You could display your stuff as if it’s a table and then vertically center content which works well but it just doesn’t feel right. It starts to feel plain wrong once you’ve tried to do this with Flexbox.

So in this example I’ve set up a container and inside of that container is an image. Flexbox is used to center the image both vertically and horizontally inside of it’s containing element. The property that is used for vertical centering is  align-items . The property that is centering horizontally in this example is  justify-content . In my opinion, this is the best way to vertically align items I’ve seen.

Fitting things into a container

The filmstrip example gave this one away already but Flexbox can be used to fit an unknown number of items into a container. This is really nice if you have a couple of images but you can’t really be sure of how many. You could optimize for a certain number, let’s say four in the case of this example. And then for the edge cases where you have five images or more, you have Flexbox to make the images smaller so everything will still fit nicely into the containing element. Let’s check it out.

All we had to do to achieve this is add  display: flex; to the containing element. In the first example we saw that, by default, children of an element with  display: flex; will shrink to fit inside of that container.

Conclusions and further resources

In this post I showed you three examples of what you can achieve with Flexbox and how you can do that. Note that we just used Flexbox for three small things and that I didn’t mention Flexbox as a method of laying out your entire page. The reason for this is that Flexbox is not intended for that. There is a spec on the way for laying out your entire page and it’s called grid.

If you’re looking for a good overview of how Flexbox works I recommend that you visit this cheatsheet on css-tricks.com. This cheatsheet provides a lot of information on how you can use Flexbox and what properties it has. Lastly, if you’re looking for more examples of what problems you can solve with Flexbox check out this “solved by Flexbox” page.