code: function isImmutable(node) {
    if (t.isType(node.type, "Immutable")) return true;
  
    if (t.isLiteral(node)) {
      if (node.regex) {
        // regexs are mutable
        return false;
      } else {
        // immutable!
        return true;
      }
    } else if (t.isIdentifier(node)) {
      if (node.name === "undefined") {
        // immutable!
        return true;
      } else {
        // no idea...
        return false;
      }
    }
  
    return false;
  }
docs: Check if the input `node` is definitely immutable.