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Una de las razones por las que flexbox atrajo rápidamente el interés de los desarrolladores web es que por primera vez en la web se ha conseguido unas posibilidades compleatas de alineamiento de elementos. Se habilita la alineación vertical, de modo que por fin existe una manera rápida y facil de centrar una caja. A lo largo de esta guía, vamos a desarrollar un exhaustivo recorrido sobre el funcionamiento de las propiedades de alineamiento y justificación en Flexbox.
To centre our box we use the
align-items property to align our item on the cross axis, which in this case is the block axis running vertically. We use
justify-content to align the item on the main axis, which in this case the inline axis running horizontally.
You can take a look at the code of this example below. Change the size of the container or nested element and the nested element always remains centered.
Properties that control alignment
The properties we will look at in this guide are as follows.
justify-content— controls alignment of all items on the main axis.
align-items— controls alignment of all items on the cross axis.
align-self— controls alignment of an individual flex item on the cross axis.
align-content— described in the spec as for “packing flex lines”; controls space between flex lines on the cross axis.
We will also discover how auto margins can be used for alignment in flexbox.
Note: The alignment properties in Flexbox have been placed into their own specification — CSS Box Alignment Level 3. It is expected that this spec will ultimately supersede the properties as defined in Flexbox Level One.
The Cross Axis
align-self properties control alignment of our flex items on the cross axis, down the columns if
row and along the row if
We are making use of cross-axis alignment in the most simple flex example. If we add
display: flex to a container, the child items all become flex items arranged in a row. They will all stretch to be as tall as the tallest item, as that item is defining the height of the items on the cross axis. If your flex container has a height set, then the items will stretch to that height, regardless of how much content is in the item.
The reason the items become the same height is that the initial value of
align-items, the property that controls alignment on the cross axis, is set to
We can use other values to control how the items align:
In the live example below, the value of
stretch. Try the other values and see how all of the items align against each other in the flex container.
Aligning one item with
align-items property sets the
align-self property on all of the flex items as a group. This means you can explicitly declare the
align-self property to target a single item. The
align-self property accepts all of the same values as
align-items plus a value of
auto, which will reset the value to that which is defined on the flex container.
In this next live example, the flex container has
align-items: flex-start, which means the items are all aligned to the start of the cross axis. I have targeted the first item using a
first-child selector and set that item to
align-items: stretch; another item has been selected using its class of
selected and given
center. You can change the value of
align-items or change the values of
align-self on the individual items to see how this works.
Changing the main axis
So far we have looked at the behaviour when our
row, and while working in a language written top to bottom. This means that the main axis runs along the row horizontally, and our cross axis alignment moves the items up and down.
If we change our
flex-direction to column,
align-self will align the items to the left and right.
You can try this out in the example below, which has a flex container with
flex-direction: column yet otherwise is exactly the same as the previous example.
Aligning content on the cross axis — the align-content property
So far we have been aligning the items, or an individual item inside the area defined by the flex-container. If you have a wrapped multiple-line flex container then you might also want to use the
align-content property to control the distribution of space between the rows. In the specification this is described as packing flex lines.
align-content to work you need more height in your flex container than is required to display the items. It then works on all the items as a set, and dictates what happens with that free space, and the alignment of the entire set of items within it.
align-content property takes the following values:
align-content: space-evenly(not defined in the Flexbox specification)
In the live example below, the flex container has a height of 400 pixels, which is more than needed to display our items. The value of
space-between, which means that the available space is shared out between the flex lines, which are placed flush with the start and end of the container on the cross axis.
Try out the other values to see how the
align-content property works.
Once again we can switch our
column in order to see how this property behaves when we are working by column. As before, we need enough space in the cross axis to have some free space after displaying all of the items.
Note: the value
space-evenly is not defined in the flexbox specification and is a later addition to the Box Alignment specification. Browser support for this value is not as good as that of the values defined in the flexbox spec.
See the documentation for
justify-content on MDN for more details on all of these values and browser support.
Aligning content on the main axis
Now that we have seen how alignment works on the cross axis, we can take a look at the main axis. Here we only have one property available to us —
justify-content. This is because we are only dealing with items as a group on the main axis. With
justify-content we control what happens with available space, should there be more space than is needed to display the items.
In our initial example with
display: flex on the container, the items display as a row and all line up at the start of the container. This is due to the initial value of
flex-start. Any available space is placed at the end of the items.
justify-content property accepts the same values as
justify-content: space-evenly(not defined in the Flexbox specification)
In the example below, the value of
space-between. The available space after displaying the items is distributed between the items. The left and right item line up flush with the start and end.
If the main axis is in the block direction because
flex-direction is set to
justify-content will distribute space between items in that dimension as long as there is space in the flex container to distribute.
Alignment and Writing Modes
Remember that with all of these alignment methods, the values of
flex-end are writing mode-aware. If the value of
start and the writing mode is left-to-right as in English, the items will line up starting at the left side of the container.
However if the writing mode is right-to-left as in Arabic, the items will line up starting at the right side of the container.
The live example below has the
direction property set to
rtl to force a right-to-left flow for our items. You can remove this, or change the values of
justify-content to see how flexbox behaves when the start of the inline direction is on the right.
Alignment and flex-direction
The start line will also change if you change the
flex-direction property — for example using
row-reverse instead of
In this next example I have items laid out with
flex-direction: row-reverse and
justify-content: flex-end. In a left to right language the items all line up on the left. Try changing
flex-direction: row-reverse to
flex-direction: row. You will see that the items now move to the right hand side.
While this may all seem a little confusing, the rule to remember is that unless you do something to change it, flex items lay themselves out in the direction that words are laid out in the language of your document along the inline, row axis.
flex-start will be where the start of a sentence of text would begin.
You can switch them to display in the block direction for the language of your document by selecting
flex-direction: column. Then
flex-start will then be where the top of your first paragraph of text would start.
If you change flex-direction to one of the reverse values, then they will lay themselves out from the end axis and in the reverse order to the way words are written in the language of your document.
flex-start will then change to the end of that axis — so to the location where your lines would wrap if working in rows, or at the end of your last paragraph of text in the block direction.
Using auto margins for main axis alignment
We don’t have a
justify-self property available to us on the main axis as our items are treated as a group on that axis. However it is possible to do some individual alignment in order to separate an item or a group of items from others by using auto margins along with flexbox.
A common pattern is a navigation bar where some key items are aligned to the right, with the main group on the left. You might think that this should be a use case for a
justify-self property, however consider the image below. I have three items on one side and two on the other. If I were able to use
justify-self on item d, it would also change the alignment of item e that follows, which may or may not be my intention.
Instead we can target item 4 and separate it from the first three items by giving it a
margin-left value of
auto. Auto margins will take up all of the space that they can in their axis — it is how centering a block with margin auto left and right works. Each side tries to take as much space as it can, and so the block is pushed into the middle.
In this live example, I have flex items arranged simply into a row with the basic flex values, and the class
margin-left: auto. You can try removing this, or adding the class to another item to see how it works.
Future alignment features for Flexbox
At the beginning of this article I explained that the alignment properties currently contained in the Level 1 flexbox specification are also included in Box Alignment Level 3, which may well extend these properties and values in the future. We have already seen one place where this has happened, with the introduction of the
space-evenly value for
The Box Alignment module also includes other methods of creating space between items, such as the
row-gap feature as seen in CSS Grid Layout. The inclusion of these properties in Box Alignment means that in future we should be able to use
row-gap in flex layouts too. This will mean you won’t need to use margins to space out flex items.
My suggestion when exploring flexbox alignment in depth is to do so alongside looking at alignment in Grid Layout. Both specifications use the alignment properties as detailed in the Box Alignment specification. You can see how these properties behave when working with a grid in the MDN article Box Alignment in Grid Layout, and I have also compared how alignment works in these specifications in my Box Alignment Cheatsheet.