--- /dev/null
+/**
+ * @fileoverview Rule to flag references to the undefined variable.
+ * @author Michael Ficarra
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+ meta: {
+ type: "suggestion",
+
+ docs: {
+ description: "disallow the use of `undefined` as an identifier",
+ category: "Variables",
+ recommended: false,
+ url: "https://eslint.org/docs/rules/no-undefined"
+ },
+
+ schema: []
+ },
+
+ create(context) {
+
+ /**
+ * Report an invalid "undefined" identifier node.
+ * @param {ASTNode} node The node to report.
+ * @returns {void}
+ */
+ function report(node) {
+ context.report({
+ node,
+ message: "Unexpected use of undefined."
+ });
+ }
+
+ /**
+ * Checks the given scope for references to `undefined` and reports
+ * all references found.
+ * @param {eslint-scope.Scope} scope The scope to check.
+ * @returns {void}
+ */
+ function checkScope(scope) {
+ const undefinedVar = scope.set.get("undefined");
+
+ if (!undefinedVar) {
+ return;
+ }
+
+ const references = undefinedVar.references;
+
+ const defs = undefinedVar.defs;
+
+ // Report non-initializing references (those are covered in defs below)
+ references
+ .filter(ref => !ref.init)
+ .forEach(ref => report(ref.identifier));
+
+ defs.forEach(def => report(def.name));
+ }
+
+ return {
+ "Program:exit"() {
+ const globalScope = context.getScope();
+
+ const stack = [globalScope];
+
+ while (stack.length) {
+ const scope = stack.pop();
+
+ stack.push(...scope.childScopes);
+ checkScope(scope);
+ }
+ }
+ };
+
+ }
+};