https://github.com/drcmda/react-spring/blob/master/src/animated/targets/react-dom/fix-auto.js

fix-auto.js

import React from 'react'
import ReactDOM from 'react-dom'
import AnimatedValue from '../../AnimatedValue'

const check = value => value === 'auto'
const convert = (acc, [name, value]) => ({
  ...acc,
  [name]: new AnimatedValue(value),
})
const overwrite = (width, height) => (acc, [name, value]) => ({
  ...acc,
  [name]: value === 'auto' ? (name === 'height' ? height : width) : value,
})

export default function fixAuto(spring, props) {
  const { native, children, from, to } = props

  // Dry-route props back if nothing's using 'auto' in there
  if (![...Object.values(from), ...Object.values(to)].some(check)) return props

  return new Promise(res => {
    const forward = spring.getForwardProps(props)
    const allProps = Object.entries({ ...from, ...to })
    const portal = document.createElement('div')
    portal.style.cssText = 'position:static;visibility:hidden;'
    document.body.appendChild(portal)

    // Collect to-state props
    const componentProps = native
      ? allProps.reduce(convert, forward)
      : { ...from, ...to, ...forward }

    // Render to-state vdom to portal
    ReactDOM.render(
      <div
        ref={ref => {
          if (ref) {
            // Once it's rendered out, fetch bounds
            const height = ref.clientHeight
            const width = ref.clientWidth

            // Remove portal and resolve promise with updated props
            document.body.removeChild(portal)
            res({
              ...props,
              from: Object.entries(from).reduce(overwrite(width, height), from),
              to: Object.entries(to).reduce(overwrite(width, height), to),
            })
          }
        }}>
        {children(componentProps)}
      </div>,
      portal
    )
  })
}