How to add code folding to output chunks in rmarkdown html documents

TOC:

1. Full control over which chunks should be folded

2. Fold all chunks that contain more than one line of code/output

---

1. Full control over which chunks should be folded

I wanted to have the same functionality as well and did the following:

I created a JavaScript that looks as follows:

$(document).ready(function() {

  $chunks = $('.fold');

  $chunks.each(function () {

    // add button to source code chunks
    if ( $(this).hasClass('s') ) {
      $('pre.r', this).prepend("<div class=\"showopt\">Show Source</div><br style=\"line-height:22px;\"/>");
      $('pre.r', this).children('code').attr('class', 'folded');
    }

    // add button to output chunks
    if ( $(this).hasClass('o') ) {
      $('pre:not(.r)', this).has('code').prepend("<div class=\"showopt\">Show Output</div><br style=\"line-height:22px;\"/>");
      $('pre:not(.r)', this).children('code:not(r)').addClass('folded');

      // add button to plots
      $(this).find('img').wrap('<pre class=\"plot\"></pre>');
      $('pre.plot', this).prepend("<div class=\"showopt\">Show Plot</div><br style=\"line-height:22px;\"/>");
      $('pre.plot', this).children('img').addClass('folded');

    }
  });

  // hide all chunks when document is loaded
  $('.folded').css('display', 'none')

  // function to toggle the visibility
  $('.showopt').click(function() {
    var label = $(this).html();
    if (label.indexOf("Show") >= 0) {
      $(this).html(label.replace("Show", "Hide"));
    } else {
      $(this).html(label.replace("Hide", "Show"));
    }
    $(this).siblings('code, img').slideToggle('fast', 'swing');
  });
});

Since I am no JS crack it might not be perfect, but it does what it is supposed to.
Include it in your Rmd file:

<script src="https://stackoverflow.com/questions/37755037/js/hideOutput.js"></script>

I also wrote some CSS definitions to style the button:

.showopt {
  background-color: #004c93;
  color: #FFFFFF; 
  width: 100px;
  height: 20px;
  text-align: center;
  vertical-align: middle !important;
  float: right;
  font-family: sans-serif;
  border-radius: 8px;
}

.showopt:hover {
    background-color: #dfe4f2;
    color: #004c93;
}

pre.plot {
  background-color: white !important;
}

After including both, the JS file and the stylesheet, you can hide chunks by wrapping a div container around them with one of the following classes:

Hide output only

<div class="fold o">
```{r}
  ...
```
</div>

Hide source code

<div class="fold s">
```{r}
  ...
```
</div>

Hide both

<div class="fold s o">
```{r}
  ...
```
</div>

The script detects the type of each chunk (e.g. source code, text output or plot output) and labels the buttons accordingly.

The result looks like this:

---

2. Fold all chunks that contain more than one line of code/output

Here is a version of the script that adds the folding feature to all chunks that are longer than one line:

$(document).ready(function() {
  $plots = $('img.plot');
  $chunks = $('pre').has('code');
  $chunks = $chunks.filter(function(idx) {
    return $(this).children('code').outerHeight(false) > parseInt($(this).css('line-height'));
  });

  $chunks.each(function () {
    if($(this).hasClass('r')) {
      $(this).append("<div class=\"showopt\">Show Source</div><br style=\"line-height:22px;\"/>");
    } else {
      $(this).append("<div class=\"showopt\">Show Output</div><br style=\"line-height:22px;\"/>");
    }
  });

  $plots.each(function () {
    $(this).wrap('<pre class=\"plot\"></pre>');
    $(this).parent('pre.plot').prepend("<div class=\"showopt\">Show Plot</div><br style=\"line-height:22px;\"/>");
  });

  // hide all chunks when document is loaded
  $chunks.children('code').toggle();
  $('pre.plot').children('img').toggle();
  // function to toggle the visibility
  $('.showopt').click(function() {
    var label = $(this).html();
    if (label.indexOf("Show") >= 0) {
      $(this).html(label.replace("Show", "Hide"));
    } else {
      $(this).html(label.replace("Hide", "Show"));
    }
    $(this).siblings('code, img').slideToggle('fast', 'swing');
  });
});

Just include it with <script src="https://stackoverflow.com/questions/37755037/js/hideAll.js"></script> and you don’t need to wrap div containers around your code chunks.
One thing you have to add in your Rmd document though is the following global chunk option:

```{r, echo = F}
knitr::opts_chunk$set(out.extra="class="plot"")
```

It is needed to identify graphical output.