offset

import {offset} from '@floating-ui/dom';

Usage

computePosition(referenceEl, floatingEl, {
  middleware: [offset(10)],
});

The value(s) passed are logical, meaning their effect on the physical result is dependent on the placement, writing direction (e.g. RTL), or alignment.

Order

offset() should generally be placed at the beginning of your middleware array.

Options

These are the options you can pass to offset().

type Options =
  | number
  | {
      mainAxis?: number;
      crossAxis?: number;
      alignmentAxis?: number | null;
    };

A number represents the distance (gutter or margin) between the floating element and the reference element. This is shorthand for mainAxis.

offset(10);

An object can also be passed, which enables you to individually configure each axis.

mainAxis

default: 0

The axis that runs along the side of the floating element. Represents the distance (gutter or margin) between the floating element and the reference element.

offset({
  mainAxis: 10,
});

Here’s how it looks on the four sides:

crossAxis

default: 0

The axis that runs along the alignment of the floating element. Represents the skidding between the floating element and the reference element.

offset({
  crossAxis: 20,
});

Here’s how it looks on the four sides:

alignmentAxis

default: null

The same axis as crossAxis but applies only to aligned placements and inverts the end alignment. When set to a number, it overrides the crossAxis value.

A positive number will move the floating element in the direction of the opposite edge to the one that is aligned, while a negative number the reverse.

offset({
  alignmentAxis: 20,
});

Here’s how it differentiates from crossAxis:

Creating custom placements

While you can only choose 12 different placements as part of the core library, you can use the offset() middleware to create any placement you want.

For example, although the library doesn’t provide a placement for centering on both axes, offset enables this via the function option by allowing you to read the rects:

computePosition(referenceEl, floatingEl, {
  middleware: [
    // Assumes placement is 'bottom' (the default)
    offset(({rects}) => {
      return (
        -rects.reference.height / 2 - rects.floating.height / 2
      );
    }),
  ],
});

In this case, the function option starts from the default bottom placement, then using that starting point, returns an offset to center the floating element on both axes.

A diagonal placement is also possible:

computePosition(referenceEl, floatingEl, {
  placement: 'top-start',
  middleware: [
    offset(({rects}) => ({
      alignmentAxis: -rects.floating.width,
    })),
  ],
});

This time, 'top-start' was used as the starting point.

So, it’s straightforward to allow this:

computePosition(referenceEl, floatingEl, {
  placement: 'center',
});

With a wrapper, like this:

import {computePosition as base, offset} from '@floating-ui/dom';
 
const centerOffset = offset(({rects}) => {
  return -rects.reference.height / 2 - rects.floating.height / 2;
});
 
export function computePosition(
  referenceEl,
  floatingEl,
  options,
) {
  const isCentered = options.placement === 'center';
  const placement = isCentered ? 'bottom' : options.placement;
  const middleware = [
    isCentered && centerOffset,
    ...(options.middleware || []),
  ];
 
  return base(referenceEl, floatingEl, {
    ...options,
    placement,
    middleware,
  });
}