Convert 'rgb()' string to hex string
While the given function does work for simple cases, it will not perform as expected if there are multiple colors within richText. The first color will override all other colors.

First, a look at your implementation:


Why is richText used in some places and the (global?) text in others? It looks like these are expected to be equivalent, so you should just use richText since it is explicitly passed in.
Instead of mutating the global text, you should return the replaced result. This makes testing the function easier and debugging much less painful.
Why is this a class function? It seems to have well defined inputs &amp; outputs that does not depend on the state of any object.
Instead of manually padding numbers with element.length === 1 ? '0' + element : element, you should use element.padStart(2, '0') if possible. The polyfill isn't that large if it isn't supported everywhere you need it yet (IE), and it is considerably easier to read.
parseInt ignores leading whitespace, so there's no need to do str.trim().
String.prototype.replace accepts a replace function, which will be called with each match. Use this to extract the rgb values instead of doing re.exec yourself. In the vast majority of cases, you should not need re.exec.
Should text inside the element really be converted? If you are manipulating HTML as text, you probably shouldn't be. If this rgb() text is in the style element of the element, you could be using element.getAttribute('style') instead.
What should happen if I wrote rgb(1, 2, 3, 4, 5, 'asdf')?


Here is how I would write this function, keeping your assumption that all rgb strings are well formed.



function cleanupRichText(richText) {
  return richText.replace(/rgb\((.+?)\)/ig, (_, rgb) =&gt; {
    return '#' + rgb.split(',')
      .map(str =&gt; parseInt(str, 10).toString(16).padStart(2, '0'))
      .join('')
  })
}

const tests = [
  ['rgb(234, 112, 30)', '#ea701e'],
  ['rgb(0, 0, 0)', '#000000'],
  ['RGB(255, 255,255)', '#ffffff'],
  ['rgb(0, 0, 0) rgb(170, 170, 170)', '#000000 #aaaaaa']
]

for (const [text, expected] of tests) {
  const result = cleanupRichText(text)
  console.log(result === expected ? 'PASS' : 'FAIL', text, '-&gt;', result)
}