Why Flexbox?
For a long time, the only reliable cross browser-compatible tools available for creating CSS layouts were things like floats and positioning. These are fine, and they work, but in some ways they are also rather limiting and frustrating.
The following simple layout requirements are either difficult or impossible to achieve with such tools, in any kind of convenient, flexible way:
- Vertically centering a block of content inside its parent.
- Making all the children of a container take up an equal amount of the available width/height, regardless of how much width/height is available.
- Making all columns in a multiple-column layout adopt the same height even if they contain a different amount of content.
As you'll see in subsequent sections, flexbox makes a lot of layout tasks much easier. Let's dig in!
The flex model
When elements are laid out as flex items, they are laid out along two axes:
- The main axis is the axis running in the direction the flex items are being laid out in e.g. as rows across the page, or columns down the page. The start and end of this axis are called the main start and main end.
- The cross axis is the axis running perpendicular to the direction the flex items are being laid out in. The start and end of this axis are called the cross start and cross end.
- The parent element that has display: flex set on it the section in our example is called the flex container.
- The items being laid out as flexible boxes inside the flex container are called flex items
If “regular” layout is based on both block and inline flow directions, the flex layout is based on “flex-flow directions”. Please have a look at this figure from the specification, explaining the main idea behind the flex layout.
Properties for the Parent
(flex container)
flex-direction
This establishes the main-axis, thus defining the direction flex items are placed in the flex container. Flexbox is (aside from optional wrapping) a single-direction layout concept. Think of flex items as primarily laying out either in horizontal rows or vertical columns.
flex-wrap
By default, flex items will all try to fit onto one line. You can change that and allow the items to wrap as needed with this property.
- nowrap (default): all flex items will be on one line
- wrap: flex items will wrap onto multiple lines, from top to bottom.
- wrap-reverse: flex items will wrap onto multiple lines from bottom to top.
justify-content
This defines the alignment along the main axis. It helps distribute extra free space leftover when either all the flex items on a line are inflexible, or are flexible but have reached their maximum size. It also exerts some control over the alignment of items when they overflow the line.
- flex-start (default): items are packed toward the start of the flex-direction.
- flex-end: items are packed toward the end of the flex-direction.
- start: items are packed toward the start of the writing-mode direction.
- end: items are packed toward the end of the writing-mode direction.
- left: items are packed toward left edge of the container, unless that doesn’t make sense with the flex-direction, then it behaves like start
- right: items are packed toward right edge of the container, unless that doesn’t make sense with the flex-direction, then it behaves like start.
- center: items are centered along the line
- space-between: items are evenly distributed in the line; first item is on the start line, last item on the end line
- space-around: items are evenly distributed in the line with equal space around them. Note that visually the spaces aren’t equal, since all the items have equal space on both sides. The first item will have one unit of space against the container edge, but two units of space between the next item because that next item has its own spacing that applies.
- space-evenly: items are distributed so that the spacing between any two items (and the space to the edges) is equal.
Properties for the Children
(flex items)
order
By default, flex items are laid out in the source order. However, the order property controls the order in which they appear in the flex container.
flex-grow
This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.
If all items have flex-grow set to 1, the remaining space in the
container will be distributed equally to all children. If one of the
children has a value of 2, the remaining space would take up twice as
much space as the others (or it will try to, at least).
Negative numbers are invalid.
flex-shrink
This defines the ability for a flex item to shrink if necessary.
flex-basis
This defines the default size of an element before the remaining space is distributed. It can be a length (e.g. 20%, 5rem, etc.) or a keyword. The auto keyword means “look at my width or height property” (which was temporarily done by the main-size keyword until deprecated). The content keyword means “size it based on the item’s content” – this keyword isn’t well supported yet, so it’s hard to test and harder to know what its brethren max-content, min-content, and fit-content do.
flex
This is the shorthand for flex-grow, flex-shrink and flex-basis combined. The second and third parameters (flex-shrink and flex-basis) are optional. The default is 0 1 auto, but if you set it with a single number value, it’s like 1 0.
align-self
This allows the default alignment (or the one specified by align-items) to be overridden for individual flex items.