Richtext

/**
* Returns definition, example and synonym
* 0 = nothing | 1 = Bold | 2 = Italic | 3 = Underline | 4 = Strikethrough
* Example: =RICHTEXT(,"RemcoE33", 1, " made this", 2)
*
* @param {"RemcoE33", 1, " made this", 2} args
* @return {string} Ritchtext string.
* @customfunction
*/
function RICHTEXT(...args){
  if(args.length % 2 != 0){
    throw new Error('You probably forgot an formatting option')
  }
  return args.join("|");
}

function setRichText_(args, e) {
  const string = args.reduce((acc, curr, i) => {
    if (i % 2 == 0){
      acc += curr
    }
    return acc
  },"");

  console.log(string)

  let richText = SpreadsheetApp.newRichTextValue()
    .setText(string)

  for(let i = 0; i < args.length; i += 2){
    const str = args[i]
    const type = args[i + 1]
    const start = string.indexOf(str)
    const end = start + str.length
    console.log(str, type, start, end)
    switch (type) {
      case '1':
        const bold = SpreadsheetApp.newTextStyle().setBold(true).build()
        richText.setTextStyle(start, end, bold)
        break;
      case '2':
        const italic = SpreadsheetApp.newTextStyle().setItalic(true).build()
        richText.setTextStyle(start, end, italic)
      break;
      case '3':
        const underline = SpreadsheetApp.newTextStyle().setUnderline(true).build()
        richText.setTextStyle(start, end, underline)
      break;
      case '4':
        const strike = SpreadsheetApp.newTextStyle().setStrikethrough(true).build()
        richText.setTextStyle(start, end, strike)
      break;
    }
  };

  e.range.setRichTextValue(richText.build())
}

function onEdit(e){
  const range = e.range;
  const formula = range.getFormula()
  const value = range.getValue()
  if(formula.match(/=richtext/gmi) && value != '#ERROR!'){
    const values = value.split("|")
    console.log(values)
    setRichText_(values, e)
  }
}