A Complete Guide to Flexbox

Background

The main idea behind the flex layout is to give the container the ability to alter its items' width/height (and order) to best fill the available space (mostly to accommodate to all kind of display devices and screen sizes). A flex container expands items to fill available free space, or shrinks them to prevent overflow.

Flexbox layout is most appropriate to the components of an application, and small-scale layouts, while the Grid layout is intended for larger scale layouts.

Basic & Terminology

Since flexbox is a whole module and not a single property, it involves a lot of things including its whole set of properties. Some of them are meant to be set on the container (parent element, known as "flex container") whereas the others are meant to be set on the children (said "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.

Basically, items will be laid out following either the main axis (from main-start to main-end) or the cross axis (from cross-start to cross-end).

  • main axis - The main axis of a flex container is the primary axis along which flex items are laid out. Beware, it is not necessarily horizontal; it depends on the flex-direction property

  • main-start | main-end - The flex items are placed within the container starting from main-start and going to main-end.

  • main size - A flex item's width or height, whichever is in the main dimension, is the item's main size. The flex item's main size property is either the ‘width’ or ‘height’ property, whichever is in the main dimension.

  • cross axis - The axis perpendicular to the main axis is called the cross axis. Its direction depends on the main axis direction.

  • cross-start | cross-end - Flex lines are filled with items and placed into the container starting on the cross-start side of the flex container and going toward the cross-end side.

  • cross size - The width or height of a flex item, whichever is in the cross dimension, is the item's cross size. The cross size property is whichever of ‘width’ or ‘height’ that is in the cross dimension.

display

This defines a flex container; inline or block depending on the given value. It enables a flex context for all its direct children.

.container {
  display: flex; /* or inline-flex */
}

Note that float, clear and vertical-align have no effect on a flex item.

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.

.item {
  order: <integer>;
}
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.

.container {
  flex-direction: row | row-reverse | column | column-reverse;
}
  • row (default): left to right in ltr; right to left in rtl
  • row-reverse: right to left in ltr; left to right in rtl
  • column: same as row but top to bottom
  • column-reverse: same as row-reverse but bottom to top
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. Direction also plays a role here, determining the direction new lines are stacked in.

.container{
  flex-wrap: nowrap | wrap | wrap-reverse;
}
  • nowrap (default): single-line / left to right in ltr; right to left in rtl
  • wrap: multi-line / left to right in ltr; right to left in rtl
  • wrap-reverse: multi-line / right to left in ltr; left to right in rtl
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, every child will set to an equal size inside the container. If you were to give one of the children a value of 2, that child would take up twice as much space as the others.

.item {
  flex-grow: <number>; /* default 0 */
}

** Negative numbers are invalid.

flex-shrink

This defines the ability for a flex item to shrink if necessary.

.item {
  flex-shrink: <number>; /* default 1 */
}

Negative numbers are invalid.

flex-flow (Applies to: parent flex container element)

This is a shorthand flex-direction and flex-wrap properties, which together define the flex container's main and cross axes. Default is row nowrap.

flex-flow: <‘flex-direction’> || <‘flex-wrap’>
flex-basis

This defines the default size of an element before the remaining space is distributed. The main-size value makes it match the width or height, depending on which is relevant based on the flex-direction.

.item {
  flex-basis: <length> | auto; /* default auto */
}

.container {
  justify-content: flex-start | flex-end | center | space-between | space-around;
}
  • flex-start (default): items are packed toward the start line
  • flex-end: items are packed toward to end line
  • 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.
    justify-content

This defines the alignment along the main axis. It helps distribute extra free space left over 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

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. Default is 0 1 auto.

.item {
  flex: none | [ <'flex-grow'> <'flex-shrink'>? || <'flex-basis'> ]
}

It is recommended that you use this shorthand property rather than set the individual properties. The short hand sets the other values intelligently.

align-items

This defines the default behaviour for how flex items are laid out along the cross axis on the current line. Think of it as the justify-content version for the cross-axis (perpendicular to the main-axis).

.container {
  align-items: flex-start | flex-end | center | baseline | stretch;
}
  • flex-start：交叉轴的起点对齐。
  • flex-end：交叉轴的终点对齐。
  • center：交叉轴的中点对齐。
  • baseline: 项目的第一行文字的基线对齐。
  • stretch（默认值）：如果项目未设置高度或设为auto，将占满整个容器的高度。