Function mergeStyles

packages/@react-spectrum/s2/style/runtime.ts:53–82

(...styles: (StyleString | null | undefined)[])

 *   // merged has `padding: 16` and `color: heading`.
 */
export function mergeStyles(...styles: (StyleString | null | undefined)[]): StyleString {
  let definedStyles = styles.filter(Boolean) as StyleString[];
  if (definedStyles.length === 1) {
    let first = definedStyles[0];
    if (typeof first !== 'string') {
      // static macro has a toString method so that we generate the style macro map for the entry
      // it's automatically called in other places, but for our merging, we have to call it ourselves
      return (first as StyleString).toString() as StyleString;
    }
    return first;
  }

  let map = new Map<string, string>();

  for (let style of definedStyles) {
    // must call toString here for the static macro
    let str = style.toString();

    for (let [k, v] of parse(str)) {
      map.set(k, v);
    }
  }

  let res = '';
  for (let value of map.values()) {
    res += value;
  }

  return res as StyleString;
}

function parse(s: string) {
  let properties = new Map<string, string>();