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CSS Flexbox 101- Introduction and sample patterns

Updated: Oct 14th, 16'

CSS Flexbox is a layout module for arranging and aligning elements on the page. I'll admit, I'm late to the CSS Flexbox party myself, having finally decided to step into the Flexbox ring just a few weeks ago. There were many reasons that sidelined me for so long- flaky browser support, divergent syntaxes, to just plain old "laziness"; CSS Flexbox to me offered nothing new I thought that existing CSS -albeit sometimes with duck tape and a few screws- can't solve already. After setting aside an afternoon to fully explore the world of CSS Flex Boxes, I return a changed man on the new module and its place in CSS. CSS Flexbox now enjoys excellent browser support, has a unified syntax, and in fact should be learned if you're lazy, as it just makes so many common tasks in CSS that are head shakingly difficult to do, well, easy and intuitive. And what you hear about CSS Flexbox being confusing to grasp- that turned out to be a myth too.

In this tutorial, I'll go over the key elements that make up Flexbox, and also show you some common Flexbox patterns to validate its usefulness. After this leisurely read you'll wonder how you ever went by so long without flexing!

It all starts with display: flex

The very first step to triggering the Flexbox layout for an element is by adding display:flex to it, which turns that element into a Flexbox container. Lets do just that with the following HEADER element:

<!-- Base HTML -->
<header class="topheader">

	<nav class="left">
		<a href="#">Home</a>
		<a href="#">News</a>
		<a href="#">About</a>
	</nav>
	
	<form>
		<input type="text" /> <input type="submit" value="Search" />
	</form>

</header>
<!-- CSS -->
.topheader{
	display: flex; /* turn topheader into flex container! */
	background: #E3FFD2;
	padding: 10px 5px;
}

.topheader nav.left a{
	padding: 10px;
	display: inline-block;
}


Figure 1.a: Without "display: flex"



Figure 1.b: With "display: flex"

As soon as we add "display:flex" to an element, the browser's inclination is to try and display all of its children elements (aka Flex Items) on one line. Our header in this case contains two flex items- a <nav> element with some links inside, and a <form> containing a search box. The two screenshots above illustrates the transformation- by default without using CSS flex box, the two children elements are displayed stacked (unless we turn to the dark side and float them, for example), but with "display:flex" added to the header, these elements appear side by side instead.

When discussing Flexbox, it's helpful to get some key terminology that you'll encounter in the rest of this article out of the way first:

When an element has "display:flex" added to it, that element becomes a Flex Container. Children elements inside it become flex items. Finally, flex items inside a flex container can be positioned along either the main axis (horizontal by default for Western, left to right languages) or the cross axis (vertical by default for Western, left to right languages).

Ok, we'll now officially ready to unlock the power of Flex Boxes!

Aligning content on the main axis (ie: horizontally): justify-content property

So the standard behaviour as soon as we turn an element into a Flex container (by adding display:flex) to it is to display all of its flex items (children elements) on one line. From here on, we can align these flex items in a variety of ways that will alone prove Flex Box's worthiness. To align flex items along the main axis (horizontal for English documents by default) inside a Flex Container, we use the following property inside the Flex Container itself:

justify-content: flex-start (default) | flex-end | center | space-between | space-around 

The default value for justify-content is "flex-start", which causes all the flex items to be aligned at the start of the Flex Container (Figure 1.b); in a horizontal layout, that means to the very left. Jumping back to our initial Flex Box example above, lets see how justify-content can easily left align the child <nav> element, but right align the <form> so they appear on two ends of the spectrum inside the Flex container:

<!-- CSS -->
.topheader{
	display: flex;
	justify-content: space-between; /* separate the kids! */
	background: #E3FFD2;
	padding: 10px 5px;
}

.topheader nav.left a{
	padding: 10px;
	display: inline-block;
}


Figure 2.a: Two flex items being pushed apart using justify-content: space-between

Got your attention perhaps? By merely using CSS Flex Box's justify-content property and setting it to space-between, we've created a common header pattern that normally would have necessitated floating left and right the two children elements, not to mention setting the parent container's overflow to "hidden" to avoid content spillage. The "space-between" value basically causes the container's flex items to be distributed with even space in between them on the same line, and with no space at either ends of the flex container. This becomes more apparent if we have more than 2 children elements. Lets add a 3rd child element to our header- how about a logo sandwiched between the original two children:

<!-- HTML: Flex container with 3 flex items -->
<header class="topheader">

	<nav class="left">
		<a href="#">Home</a>
		<a href="#">News</a>
		<a href="#">About</a>
	</nav>

	<a href="#"><img src="jklogosmall.gif" /></a>
	
	<form>
		<input type="text" /> <input type="submit" value="Search" />
	</form>

</header>


Figure 2.b: Three flex items spaced apart evenly using justify-content: space-between

Notice the even space between the 3 flex items, with no space at the start or end of the flex container. And btw, it looks like we've just inadvertently created another common header pattern- content on the left, logo in the center, followed by content on the right, using nothing but justify-content. And we're just getting started!

-Possible Values

So the justify-content property aligns flex items inside it on the main axis (horizontal for English documents). The following briefly describes all the possible values for it:

  • flex-start: Flex items are aligned at the start of the flex container. Default value.
  • flex-end: Flex items are aligned at the end of the flex container.
  • center: Flex items are aligned at the center of the flex container, flushed. Any left over space is evenly distributed at the beginning and end of the flex container.
  • space-between: Flex items are distributed evenly across the flex container. The first flex item is aligned at the beginning of the container, and the last item at the very end.
  • space-around: Flex items are distributed evenly across the flex container, with the very first and very last flex item also carrying space (1/2 of that of the other flex items) at the beginning and very end of the flex container, respectively.

Try it Yourself

The best way to understand the difference between the above values is to actually try them out. Step into the playground below to see how the different values of justify-content affect a flex container with 3 flex items inside it:


Playground: "justify-content" property

Aligning flex items on the cross axis (ie: vertically): align-items property

So far we've learned that a Flex Container by default will try to display all of its flex items (children elements) on a single line, with the main axis (ie: horizontal) alignment of the flex items controlled by justify-content. What about vertical alignment? That's where the align-items property comes in- it controls the cross axis (vertical in English documents by default) spacing between flex items on the same line:

align-items: flex-start | flex-end | center | baseline | stretch (default) 

Our menu header example with the logo in the middle looks good already, but it might look even better if all of the children elements are vertically centered. This can easily be done by setting align-items to "center":

<!-- CSS -->
.topheader{
	display: flex;
	justify-content: space-between;
	align-items: center; /* center flex items vertically */
	background: #E3FFD2;
	padding: 10px 5px;
}

.topheader nav.left a{
	padding: 10px;
	display: inline-block;
}


Figure 3: Vertically centering flex items on the same line using align-items: center

This causes the three flex items inside the Flex Container- the <nav>, logo, and <form> to be centered vertically on the same line.

-Possible Values

As mentioned, the align-items property aligns flex items inside the flex container along the cross axis (vertical for English documents by default). The following briefly describes all the possible values for it:

  • flex-start: Flex items are aligned at the top of the flex container.
  • flex-end: Flex items are aligned at the bottom of the flex container.
  • center: Flex items are aligned at the center of the flex container.
  • baseline: Aligns the flex items by their respective baselines- usually, this means at the top of the flex container.
  • stretch: Stretches the height of all flex items so they all align vertically. Default value.

The default value is "stretch", which causes flex items with uneven heights to stretch so they match up with that of the greatest height. And right there by the way it how Flex Box allows us to easily create multiple column layouts with equal heights, without doing anything other than enable Flex Box on the parent container of the columns (display:flex).

Try it Yourself

Click below to see how the align-items property affects a flex container with 3 flex items of differing heights:


Playground: "align-items" property

Aligning a specific flex item on the cross axis (ie: vertically): align-self property

We just saw the align-items property, which aligns flex items on the same line on the cross axis (ie: vertically). Along the same lines (no pun intended) is the align-self property, which works exactly like align-items, though is added inside specific flex items to affect cross axis alignment for those flex items specifically, deviating from the setting set by align-items on the parent flex container.

-Possible Values: Same as those for align-items above

Lets say you use align-items to vertically center 4 flex items inside a flex container, but for the first flex item, you want to break away from that setting and align it at the bottom of the line. Simply add align-self inside the target flex item and set it to "flex-end".

Try it Yourself

Click below to see how the align-self property is used to set the vertical alignment of a specific flex item to be different from its peers:


Playground: "align-self" property

Aligning a specific flex item on the main axis (ie: horizontally): margin-left and margin-right property

So for aligning flex elements vertically inside a flex container, we can use align-items, which affects all of its flex items in general, or align-self to target specific/ individual flex items.  A natural question to ask at this point is, what about alignment on  the main (ie: horizontal) axis? Does there also exist a "justify-self" property to accompany justify-content to align specific elements on the main axis differently from its peers? Surprisingly, there isn't, though we could use margin-left and margin-right instead to accomplish the same thing in many respects, apparently:

  • Add margin-left: auto inside a specific flex element to right align it and all flex elements that follow it inside the flex container
  • Add margin-right: auto inside a specific flex element to left align it and all flex elements that proceed it inside the flex container

Here are some examples of adding margins to specific flex elements inside a flex container to control individual horizontal alignment:

Try it Yourself

Click below to play with the margin-left and margin-right properties to set the vertical alignment of specific flex item in a flex container:


Playground: "margin" property

Flex Items direction: flex-direction property

Near the start of this tutorial, I mentioned that the standard behaviour of a Flex Container is to display all of its flex items (children elements) on one line, similar to floating these elements. This is due to the default setting for the next property I'm about to introduce, flex-direction. This property controls the flow of the flex items within a flex container. The default value for flex-direction is "rows", though with a change to "column", we can position the flex items as individual columns stacked one against the other instead. Here's our header example with 3 flex items, but this time, stacked together instead of side by side:

<!-- CSS -->
.topheader{
	display: flex;
	justify-content: space-between;
	align-items: center;
	flex-direction: column; /* stack the flex items instead of all on same line */
	background: #E3FFD2;
	padding: 10px 5px;
}

.topheader nav.left a{
	padding: 10px;
	display: inline-block;
}


Figure 4: Changing to vertical/column layout using flex-direction: column

The three flex items now appear one below the next, each occupying the entire horizontal space.

Important: When you set the layout of a Flex Container to vertical (flex-direction:column), the role of the justify-content and align-items properties reverses as the main and cross axis inside the flex container are reversed. justify-content now controls the vertical alignment of the flex items, and align-items the horizontal alignment (for English based documents).

-Possible Values

flex-direction as mentioned controls the direction of the flex items within the flex container, whether horizontal ('row') or vertical ('column'). Here are the possible values:

  • row: Flex items are positioned horizontally on the same line.
  • row-reverse: Flex items are positioned horizontally on the same line, but with the order of the flex items reversed.
  • column: Flex items are positioned vertically, stacked.
  • column-reverse: Flex items are positioned vertically, stacked, but with the order of the flex items reversed.

Try it Yourself

Launch the flex-direction playground below to experiment with the different values, and also, to see how the roles of justify-content and align-items properties in this case are reversed.


Playground: "flex-direction" property

Specifying Flex Items wrap: flex-wrap property

Flex items within a flex container by default do not wrap- when we resize the browser, the dimensions of each flex item shrinks as well to try and stay on the same line. When a flex item can no longer shrink any further (ie: due to an image with a set width or a long word), it starts to overflow the container. We can change this behaviour to get flex items to wrap to the next line instead in this case, by setting flex-wrap to "wrap".

Bringing in our familiar header example again, the original behavior if we decrease the width of the <header> container is for its children elements to also decrease in size, for example:


Figure 5a: Original behavior of a flex container with 3 flex items- all 3 items stay on one line regardless of parent container size

The 3 children elements like little duckies stay on the same line regardless of how small the parent flex container (the links inside the first flex item may spill over to another line, but the flex item itself doesn't). By setting flex-wrap to 'wrap', we can give our duckies permission to break rank and jump to the next line instead of reducing their intrinsic width/height to fit on the same line:

<!-- CSS -->
.topheader{
	display: flex;
	justify-content: space-between;
	align-items: center;
	flex-wrap: wrap;
	background: #E3FFD2;
	padding: 10px 5px;
}

.topheader nav.left a{
	padding: 10px;
	display: inline-block;
}


Figure 5b: With flex-wrap: wrap, flex items maintain their intrinsic widths and spill over to next line if parent container is too small

-Possible Values

flex-wrap supports the following 3 values:

  • no-wrap: All flex items inside the flex container are positioned on the same line. Default value.
  • wrap: Flex items wrap if needed to become multi-lined.
  • wrap-reverse: Like 'wrap' above, but flex items wrap up instead of down to the next line.

Try it Yourself

Launch the flex-wrap playground to see the behavior of flex items based on whether flex-wrap is set to "wrap" or "nowrap":


Playground: "flex-wrap" property

Aligning multi-lined flex items: align-content property

When flex items are allowed to wrap onto multiple lines (using flex-wrap:wrap), we need another flex property to control the alignment of these flex items relative to the flex container on the cross axis (ie: vertically). For that we have align-content. We've already seen the align-items property, which handles cross axis alignment of flex items on the same line relative to each other. align-content on the other hand does the same for multiple lines of flex items, and relative to the flex container.

To see the often times subtle difference between align-content and align-items properties, take a look at the following two images:


Figure 6a: "align-content:spacearound" ONLY

Figure 6b: align-content:spacearound AND
align-items:center

Can you spot the difference? The first figure 6a uses only align-content and set to "space-around" to space the multi-lined flex items equally in the cross axis (vertically) inside the flex container. It doesn't affect flex items on the same line in any way, which is why flex item #2 isn't centered relative to items #3 and #4 on the same line. In figure 6b, we also add the align-items property, which deals with alignment of flex items on the same line. By setting it to "center", flex items #2, #3, and #4 are now also centered vertically in relationship to each other.

So to recap, align-content is used to align flex items that are allowed to wrap and span multiple lines relative to the flex container on the cross axis (ie: vertically). align-items on the other hand is used for alignment of flex items relative to its peers on the same line on the cross axis (ie: vertically).

-Possible Values

The align-content property takes on the same values as those for justify-content, namely:

  • flex-start: Flex items are aligned at the start of the flex container on the cross axis. Default value.
  • flex-end: Flex items are aligned at the end of the flex container.
  • center: Flex items are aligned at the center of the flex container, flushed. Any left over space is evenly distributed at the beginning and end of the flex container.
  • space-between: Flex items are distributed evenly across the flex container. The first flex item is aligned at the beginning of the container, and the last item at the very end.
  • space-around: Flex items are distributed evenly across the flex container, with the very first and very last flex item also carrying space (1/2 of that of the other flex items) at the beginning and very end of the flex container, respectively.

Try it Yourself

To experiment with the difference between align-content and align-items yourself, open the align-content playground below:


Playground: "align-content" property

Setting Flex Items Dimensions and order

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