@@ -1,3 +1,8 @@
 {
-  "extends": "./node_modules/@microsoft/rush-stack-compiler-3.5/includes/tsconfig-node.json"
+  "extends": "./node_modules/@microsoft/rush-stack-compiler-3.5/includes/tsconfig-node.json",
+  "compilerOptions": {
+    "types": [
+      "node"
+    ]
+  }
 }
@@ -4,6 +4,7 @@
 import { TSESLint } from '@typescript-eslint/experimental-utils';
 
 import { noNullRule } from './no-null';
+import { noUntypedUnderscoreRule } from './no-untyped-underscore';
 
 interface IPlugin {
   rules: { [ruleName: string]: TSESLint.RuleModule<string, unknown[]> };
@@ -12,7 +13,8 @@ interface IPlugin {
 const plugin: IPlugin = {
   rules: {
     // NOTE: The actual ESLint rule name will be "@rushstack/no-null".
-    'no-null': noNullRule
+    'no-null': noNullRule,
+    'no-untyped-underscore': noUntypedUnderscoreRule
   }
 };
 
 
@@ -26,9 +26,9 @@ const noNullRule: TSESLint.RuleModule<MessageIds,Options> = {
   },
   create: (context: TSESLint.RuleContext<MessageIds, Options>) => {
     return {
-      Literal: function(node: TSESTree.Node) {
+      Literal: function(node: TSESTree.Literal) {
         // Is it a "null" literal?
-        if (node.type === 'Literal' && node.value === null) {
+        if (node.value === null) {
 
           // Does the "null" appear in a comparison such as "if (x === null)"?
           let isComparison: boolean = false;
 
@@ -0,0 +1,66 @@
+// Copyright (c) Microsoft Corporation. All rights reserved. Licensed under the MIT license.
+// See LICENSE in the project root for license information.
+
+import {
+  TSESTree,
+  TSESLint
+} from '@typescript-eslint/experimental-utils';
+
+type MessageIds = 'error-untyped-underscore';
+type Options = [ ];
+
+const noUntypedUnderscoreRule: TSESLint.RuleModule<MessageIds,Options> = {
+  meta: {
+    type: 'problem',
+    messages: {
+      'error-untyped-underscore':
+        'This expression appears to access a private member "{{memberName}}"; '
+        + 'either remove the underscore prefix or else declare a type for the containing object'
+    },
+    schema: [ ],
+    docs: {
+      description: 'Prevents usage of JavaScript\'s "null" keyword.',
+      category: 'Stylistic Issues',
+      recommended: "error",
+      url: 'https://www.npmjs.com/package/@rushstack/eslint-config'
+    }
+  },
+  create: (context: TSESLint.RuleContext<MessageIds, Options>) => {
+    return {
+      MemberExpression: function(node: TSESTree.MemberExpression) {
+        // Is it an expression like "x.y"?
+        let match: boolean = true;
+
+        // Ignore expressions such as "super.y", "this.y", and "that.y"
+        const memberObject: TSESTree.LeftHandSideExpression = node.object;
+        if (memberObject) {
+          if (memberObject.type === 'Super' || memberObject.type === 'ThisExpression') {
+            match = false;
+          } else {
+            if (memberObject.type === 'Identifier') {
+              if (memberObject.name === 'this' || memberObject.name == 'that') {
+                match = false;
+              }
+            }
+          }
+        }
+
+        // Does the member name start with an underscore?  (e.g. "x._y")
+        if (match && node.property && node.property.type === 'Identifier') {
+          const memberName: string = node.property.name;
+          if (memberName && memberName[0] === '_') {
+            context.report({
+              node,
+              messageId: 'error-untyped-underscore',
+              data: {
+                memberName: memberName
+              }
+            });
+          }
+        }
+      }
+    };
+  }
+};
+
+export { noUntypedUnderscoreRule };
Original file line number	Diff line number	Diff line change
`@@ -1,3 +1,8 @@`
`1`	`1`	`{`
`2`		`- "extends": "./node_modules/@microsoft/rush-stack-compiler-3.5/includes/tsconfig-node.json"`
	`2`	`+ "extends": "./node_modules/@microsoft/rush-stack-compiler-3.5/includes/tsconfig-node.json",`
	`3`	`+ "compilerOptions": {`
	`4`	`+ "types": [`
	`5`	`+ "node"`
	`6`	`+ ]`
	`7`	`+ }`
`3`	`8`	`}`