How to use the postcss-scss.stringify function in postcss-scss

root.walkAtRules('mixin', (atRule) => {
    const { params } = atRule;
    const selector = mixinParamsToFunc(params);

    let contents = '';
    postcssScss.stringify(atRule, (string, node, startOrEnd) => {
      // if node.type === decl skip when doing this above
      // stops first and last part entering the string e.g "@mixin ad-exact($width, $height) {"
      if (node && node === atRule && startOrEnd) return;

      contents += string;
    });

    let isUsedInFile = false;
    // search to see if mixin is used in file
    root.walkAtRules('include', (rule) => {
      if (rule.originalParams.split('(')[0] === params.split('(')[0]) {
        isUsedInFile = true;
      }
    });

    output.classes.set(selector, {

}

    if (
      checkUpTree(
        rule,
        nodeToCheck => nodeToCheck.type === 'atrule' && nodeToCheck.name === 'mixin',
      )
    ) return;

    if (rule.contentsAlreadyPrinted) {
      return;
    }

    let contents = '';

    postcssScss.stringify(rule, (string, node, startOrEnd) => {
      if (node && node === rule && startOrEnd) return;

      // asumption here is that there is some state involved
      // e.g &:hover or &.is-selected.
      const nestedInAmpersand = node
        && checkUpTree(
          node,
          nodeToCheck => nodeToCheck.type === 'rule' && nodeToCheck.selector.startsWith('&'),
        );

      if (node && isPlaceHolder) {
        node.contentsAlreadyPrinted = true;
      }

      // ref class if nested in ampersand
      if (