src/language-js/printer-estree.js/separatorWithWhitespace
function separatorWithWhitespace(
  isFacebookTranslationTag,
  child,
  childNode,
  nextNode
) {
  if (isFacebookTranslationTag) {
    return hardline;
  }

  if (child.length === 1) {
    return (childNode.type === "JSXElement" && !childNode.closingElement) ||
      (nextNode && nextNode.type === "JSXElement" && !nextNode.closingElement)
      ? hardline
      : softline;
  }

  return hardline;
}