code: function(ast) {
      let value = this.value;
  
      ast.traverseByType('string', function(string) {
        if (string.content[0] === '"' && value === 'single') {
          string.content = string.content
              // Unescape all escaped double quotes
              .replace(/\\"/g, '"')
              // Escape all the single quotes
              .replace(/([^\\])'/g, '$1\\\'')
              // Replace the first and the last quote
              .replace(/^"|"$/g, '\'');
        } else if (string.content[0] === '\'' && value === 'double') {
          string.content = string.content
              // Unescape all escaped single quotes
              .replace(/\\'/g, '\'')
              // Escape all the double quotes
              .replace(/([^\\])"/g, '$1\\\"')
              // Replace the first and the last quote
              .replace(/^'|'$/g, '"');
        }
      });
    }
docs: Processes tree node.
  @param {node} ast