postcss-merge-longhand/src/lib/parseTrbl.js

Merge longhand properties into shorthand with PostCSS.

import { list } from 'postcss';

/** @param {string | string[]} v */
export default (v) => {
  const s = typeof v === 'string' ? list.space(v) : v;
  return [
    s[0], // top
    s[1] || s[0], // right
    s[2] || s[0], // bottom
    s[3] || s[1] || s[0], // left
  ];
};