src/language-js/print/function.js/printReturnType
function printReturnType(path, print, options) {
  const node = path.getValue();
  const returnType = print("returnType");

  if (
    node.returnType &&
    isFlowAnnotationComment(options.originalText, node.returnType)
  ) {
    return [" /*: ", returnType, " */"];
  }

  const parts = [returnType];

  // prepend colon to TypeScript type annotation
  if (node.returnType && node.returnType.typeAnnotation) {
    parts.unshift(": ");
  }

  if (node.predicate) {
    // The return type will already add the colon, but otherwise we
    // need to do it ourselves
    parts.push(node.returnType ? " " : ": ", print("predicate"));
  }

  return parts;
}