polished/lib/mixins/retinaImage.js.flow

A lightweight toolset for writing styles in Javascript.

// @flow
import hiDPI from './hiDPI'
import PolishedError from '../internalHelpers/_errors'

import type { Styles } from '../types/style'

/**
 * A helper to generate a retina background image and non-retina
 * background image. The retina background image will output to a HiDPI media query. The mixin uses
 * a _2x.png filename suffix by default.
 *
 * @example
 * // Styles as object usage
 * const styles = {
 *  ...retinaImage('my-img')
 * }
 *
 * // styled-components usage
 * const div = styled.div`
 *   ${retinaImage('my-img')}
 * `
 *
 * // CSS as JS Output
 * div {
 *   backgroundImage: 'url(my-img.png)',
 *   '@media only screen and (-webkit-min-device-pixel-ratio: 1.3),
 *    only screen and (min--moz-device-pixel-ratio: 1.3),
 *    only screen and (-o-min-device-pixel-ratio: 1.3/1),
 *    only screen and (min-resolution: 144dpi),
 *    only screen and (min-resolution: 1.5dppx)': {
 *     backgroundImage: 'url(my-img_2x.png)',
 *   }
 * }
 */
export default function retinaImage(
  filename: string,
  backgroundSize?: string,
  extension?: string = 'png',
  retinaFilename?: string,
  retinaSuffix?: string = '_2x',
): Styles {
  if (!filename) {
    throw new PolishedError(58)
  }
  // Replace the dot at the beginning of the passed extension if one exists
  const ext = extension.replace(/^\./, '')
  const rFilename = retinaFilename
    ? `${retinaFilename}.${ext}`
    : `${filename}${retinaSuffix}.${ext}`

  return {
    backgroundImage: `url(${filename}.${ext})`,
    [hiDPI()]: {
      backgroundImage: `url(${rFilename})`,
      ...(backgroundSize ? { backgroundSize } : {}),
    },
  }
}