rgba-to-hexa.js

import * as parseCSS from 'https://tomhodgins.github.io/parse-css/index.js'

const css = parseCSS.parseAComponentValue(
  Deno.args[1].trim()
)

if (
  css.type === 'FUNCTION'
  && css.name.match(/rgba?/)
) {
  const colors = css.value.filter(({tokenType}) =>
    ['PERCENTAGE', 'NUMBER'].some(term => term === tokenType)
  )

  console.log(
    `#${colors.map(({tokenType, value}, index) => {
      // Number in the first 3 colors
      if (
        index !== 3
        && tokenType === 'NUMBER'
      ) {
        return Math.min(Math.max(0, value), 255)
      }

      // Number as the last color
      if (
        index === 3
        && tokenType === 'NUMBER'
      ) {
        return (Math.min(Math.max(0, value), 1) * 255)
      }

      // Any color that's a percentage
      if (tokenType === 'PERCENTAGE') {
        return Math.min(Math.max(0, ((value / 100) * 255)), 255)
      }
    }).map(color => Math.round(color).toString(16).padStart(2, 0)).join('')}`
  )
}

test.css

/* These are all equivalent to #808080 */

rgb(50% 50% 50% / .5)
rgb(50% 50% 50% / 50%)
rgb(128 128 128 / .5)
rgb(128 128 128 / 50%)
rgb(50%, 50%, 50%, .5)
rgb(50%, 50%, 50%, 50%)
rgb(128, 128, 128, .5)
rgb(128, 128, 128, 50%)
rgba(50% 50% 50% / .5)
rgba(50% 50% 50% / 50%)
rgba(128 128 128 / .5)
rgba(128 128 128 / 50%)
rgba(50%, 50%, 50%, .5)
rgba(50%, 50%, 50%, 50%)
rgba(128, 128, 128, .5)
rgba(128, 128, 128, 50%)