groupStyles.ts

type MergedSheet<T> = { [P in keyof T]?: T[P][] }

export function groupStyles<T>(
  sheets: Array<T>
): MergedSheet<T> {
  const merged: MergedSheet<T> = {}
  for (const sheet of sheets) {
    for (const ele in sheet) {
      if (merged[ele]) {
        merged[ele] = merged[ele] || []

        merged[ele].push(sheet[ele])
        //          ^^^^
        // [tsserver 2339] [E] Property 'push' does not exist on type 'MergedSheet<T>[Extract<keyof T, string>]'.

      } else {
        merged[ele] = [sheet[ele]]
      }
    }
  }
  return merged
}