function blockStyle()

in packages/eui/src/components/markdown_editor/markdown_actions.ts [399:512]
110 lines of code
22 McCabe index (conditional complexity)

function blockStyle(
  textarea: HTMLTextAreaElement,
  arg: StyleArgs
): SelectionRange {
  let newlinesToAppend;
  let newlinesToPrepend;

  const {
    prefix,
    suffix,
    blockPrefix,
    blockSuffix,
    replaceNext,
    prefixSpace,
    scanFor,
    surroundWithNewlines,
  } = arg;
  const originalSelectionStart = textarea.selectionStart;
  const originalSelectionEnd = textarea.selectionEnd;

  let selectedText = textarea.value.slice(
    textarea.selectionStart,
    textarea.selectionEnd
  );
  let prefixToUse =
    isMultipleLines(selectedText) && blockPrefix.length > 0
      ? `${blockPrefix}\n`
      : prefix;
  let suffixToUse =
    isMultipleLines(selectedText) && blockSuffix.length > 0
      ? `\n${blockSuffix}`
      : suffix;

  if (prefixSpace) {
    const beforeSelection = textarea.value[textarea.selectionStart - 1];
    if (
      textarea.selectionStart !== 0 &&
      beforeSelection != null &&
      !beforeSelection.match(/\s/)
    ) {
      prefixToUse = ` ${prefixToUse}`;
    }
  }
  selectedText = expandSelectedText(
    textarea,
    prefixToUse,
    suffixToUse,
    arg.multiline
  );
  let selectionStart = textarea.selectionStart;
  let selectionEnd = textarea.selectionEnd;
  const hasReplaceNext =
    replaceNext.length > 0 &&
    suffixToUse.indexOf(replaceNext) > -1 &&
    selectedText.length > 0;
  if (surroundWithNewlines) {
    const ref = newlinesToSurroundSelectedText(textarea);
    newlinesToAppend = ref.newlinesToAppend;
    newlinesToPrepend = ref.newlinesToPrepend;
    prefixToUse = newlinesToAppend + prefix;
    suffixToUse += newlinesToPrepend;
  }

  if (
    selectedText.startsWith(prefixToUse) &&
    selectedText.endsWith(suffixToUse)
  ) {
    const replacementText = selectedText.slice(
      prefixToUse.length,
      selectedText.length - suffixToUse.length
    );
    if (originalSelectionStart === originalSelectionEnd) {
      let position = originalSelectionStart - prefixToUse.length;
      position = Math.max(position, selectionStart);
      position = Math.min(position, selectionStart + replacementText.length);
      selectionStart = selectionEnd = position;
    } else {
      selectionEnd = selectionStart + replacementText.length;
    }
    return { text: replacementText, selectionStart, selectionEnd };
  } else if (!hasReplaceNext) {
    let replacementText = prefixToUse + selectedText + suffixToUse;
    selectionStart = originalSelectionStart + prefixToUse.length;
    selectionEnd = originalSelectionEnd + prefixToUse.length;
    const whitespaceEdges = selectedText.match(/^\s*|\s*$/g);
    if (arg.trimFirst && whitespaceEdges) {
      const leadingWhitespace = whitespaceEdges[0] || '';
      const trailingWhitespace = whitespaceEdges[1] || '';
      replacementText =
        leadingWhitespace +
        prefixToUse +
        selectedText.trim() +
        suffixToUse +
        trailingWhitespace;
      selectionStart += leadingWhitespace.length;
      selectionEnd -= trailingWhitespace.length;
    }
    return { text: replacementText, selectionStart, selectionEnd };
  } else if (scanFor.length > 0 && selectedText.match(scanFor)) {
    suffixToUse = suffixToUse.replace(replaceNext, selectedText);
    const replacementText = prefixToUse + suffixToUse;
    selectionStart = selectionEnd = selectionStart + prefixToUse.length;
    return { text: replacementText, selectionStart, selectionEnd };
  } else {
    const replacementText = prefixToUse + selectedText + suffixToUse;
    selectionStart =
      selectionStart +
      prefixToUse.length +
      selectedText.length +
      suffixToUse.indexOf(replaceNext);
    selectionEnd = selectionStart + replaceNext.length;
    return { text: replacementText, selectionStart, selectionEnd };
  }
}