AutoFit

Automatically fit text into its container, preventing overflow and underflow.

The algorithm uses computed width and height and therefore works for all font types and variations. It adjusts the font size of a given element so that it precisely fills its container. Not only text, it works for any elements with dimensions defined relative to font size (eg. width: 1em).

Live demo

React component AutoFit

The AutoFit component automatically re-runs when children changes and when the viewport resizes.

import { AutoFit } from 'auto-fit'

export const Title = ({ text }) => {
  return (
    <div style={{ maxWidth: '60%', margin: '0 auto' }}>
      <AutoFit>{text}</AutoFit>
    </div>
  )
}

AutoFit Props

Name	Type	Default	Description
children	ReactNode		The content to be auto fitted.
multiline	boolean	false	Allow text to wrap and fit into both container width and height.
minFontSizePx	number	8	The smallest font size the algorithm will use.
maxFontSizePx	number	160	The largest font size the algorithm will use.
as	string | ReactComponent	'div'	The underlying component that AutoFit will use.

Low-level autoFit function

For advanced use cases, the autoFit function may be useful. It is used by the AutoFit component and has no dependencies.

import { autoFit } from 'auto-fit'

autoFit(options)

If you have many elements or re-run autoFit frequently, throttle is good for performance:

import { autoFit, throttleAnimationFrame } from 'auto-fit'

const throttledAutoFit = throttleAnimationFrame(() => autoFit(options))

throttledAutoFit() // Subsequent calls are reasonably throttled

autoFit options

Name	Type	Default	Description
innerEl	HTMLElement		The inner element to be auto fitted.
containerEl	HTMLElement		The container element is used as bounding box for the inner element.
minFontSizePx	number	8	The smallest font size the algorithm will use.
maxFontSizePx	number	160	The largest font size the algorithm will use.

Details

• The single-line algorithm predicts how the browser will render text in a different font size and iterates until converging with an error of <= 0.1px (usually 1-2 iterations).
• The multi-line algorithm performs a binary search among the possible font sizes until converging with an error of <= 0.1px (usually ~10 iterations). There is no reliable way of predicting how the browser will render text in a different font size when multi-line text wrap is at play.
• Performance Each iteration has a performance hit. AutoFit uses requestAnimationFrame to throttle repeated calls, ensuring that we render as often as possible without excessively blocking the UI.
• No overflow. After usual iteration, the algorithms runs a second loop to ensure that no overflow occurs. Underflow is preferred since it doesn't look visually broken like overflow does.
• Font size is used rather than transform scale since the latter wouldn't support multi-line text wrap. Transform scale would work for singleline but we prefer to keep the two modes similar. Furthermore, transform scale tends to make text blurry in some browsers.

Developing

When developing one typically wants to see the output in the example application without having to publish and install. We achieve this by linking the local package into the example app.

Because of issues with yarn link, we use Yalc. A linking approach is preferred over yarn workspaces since we want to run the package as it would appear in the real world.

npm i yalc -g
yarn
yarn watch

# Other terminal
cd example
yarn
yalc add --link auto-fit
yarn dev

Notes on Yalc

Using --link makes it so that Next.js can HMR the updates instantly. It creates a link: rather than a file: dependency.

Publishing

# Bump version number
yarn clean && yarn build
npm publish