src/language-js/printer-estree-json.js/clean
function clean(node, newNode /*, parent*/) {
  const { type } = node;
  if (type === "Identifier") {
    return { type: "StringLiteral", value: node.name };
  }
  if (type === "UnaryExpression" && node.operator === "+") {
    return newNode.argument;
  }
}