src/language-js/print/jsx.js/separatorNoWhitespace
function separatorNoWhitespace(
  isFacebookTranslationTag,
  child,
  childNode,
  nextNode
) {
  if (isFacebookTranslationTag) {
    return "";
  }

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

  return softline;
}