Fold code blocks with Pandc Lua Filter originally posted to https://blog.atusy.net/2020/03/07/pandoc-lua-detailed-codeblock/ in Japanese

foldableCodeBlcok.lua

function CodeBlock(elem)
  if elem.classes and elem.classes:find("details") then
    local summary = "Code"
    if elem.attributes.summary then
      summary = elem.attributes.summary
    end
    return{
      pandoc.RawBlock(
        "html", "<details><summary>" .. summary .. "</summary>"
      ),
      elem,
      pandoc.RawBlock("html", "</details>")
    }
  end
end

foldableCodeBlcok.Rmd

---
output:
  html_document:
    pandoc_args: [
        "--lua-filter", "foldableCodeBlock.lua"
      ]
---

```{r, attr.output='.details summary="Output"'}
set.seed(1)
rnorm(10)
```