diff --git a/.eslintrc.json b/.eslintrc.json
index e84c8d766..3b0c8237d 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -17,7 +17,6 @@
     "@rebeccastevens/eslint-config/modern",
     "@rebeccastevens/eslint-config/typescript",
     "@rebeccastevens/eslint-config/common-overrides",
-    "plugin:eslint-plugin/recommended",
     "plugin:prettier/recommended",
     "prettier"
   ],
@@ -60,6 +59,12 @@
         "functional/no-throw-statement": "off"
       }
     },
+    // Source files.
+    {
+      "files": ["src/**/*"],
+      "extends": ["plugin:eslint-plugin/recommended"],
+      "rules": {}
+    },
     // FIXME: This override is defined in the upsteam; it shouldn't need to be redefined here. Why?
     {
       "files": ["**/*.md/**"],
diff --git a/package.json b/package.json
index d984ba070..1d70b3e53 100644
--- a/package.json
+++ b/package.json
@@ -70,7 +70,7 @@
     "@commitlint/cli": "^13.2.1",
     "@commitlint/config-conventional": "^13.2.0",
     "@istanbuljs/nyc-config-typescript": "^1.0.1",
-    "@rebeccastevens/eslint-config": "^1.1.4",
+    "@rebeccastevens/eslint-config": "^1.1.5",
     "@rollup/plugin-commonjs": "^21.0.0",
     "@rollup/plugin-json": "^4.1.0",
     "@rollup/plugin-node-resolve": "^13.0.5",
diff --git a/src/common/ignore-options.ts b/src/common/ignore-options.ts
index 5f0016faf..2df431513 100644
--- a/src/common/ignore-options.ts
+++ b/src/common/ignore-options.ts
@@ -8,6 +8,7 @@ import {
   hasID,
   hasKey,
   isAssignmentExpression,
+  isDefined,
   isPropertyDefinition,
   isExpressionStatement,
   isIdentifier,
@@ -115,34 +116,34 @@ function getNodeIdentifierText(
   node: TSESTree.Node | null | undefined,
   context: RuleContext<string, BaseOptions>
 ): string | undefined {
-  return node === undefined || node === null
-    ? undefined
-    : isIdentifier(node)
-    ? node.name
-    : hasID(node)
-    ? getNodeIdentifierText(node.id, context)
-    : hasKey(node)
-    ? getNodeIdentifierText(node.key, context)
-    : isAssignmentExpression(node)
-    ? getNodeIdentifierText(node.left, context)
-    : isMemberExpression(node)
-    ? `${getNodeIdentifierText(node.object, context)}.${getNodeIdentifierText(
-        node.property,
-        context
-      )}`
-    : isThisExpression(node)
-    ? "this"
-    : isUnaryExpression(node)
-    ? getNodeIdentifierText(node.argument, context)
-    : isExpressionStatement(node)
-    ? context.getSourceCode().getText(node)
-    : isTSArrayType(node) ||
-      isTSIndexSignature(node) ||
-      isTSTupleType(node) ||
-      isTSTypeAnnotation(node) ||
-      isTSTypeLiteral(node) ||
-      isTSTypeReference(node)
-    ? getNodeIdentifierText(node.parent, context)
+  return isDefined(node)
+    ? isIdentifier(node)
+      ? node.name
+      : hasID(node) && isDefined(node.id)
+      ? getNodeIdentifierText(node.id, context)
+      : hasKey(node) && isDefined(node.key)
+      ? getNodeIdentifierText(node.key, context)
+      : isAssignmentExpression(node)
+      ? getNodeIdentifierText(node.left, context)
+      : isMemberExpression(node)
+      ? `${getNodeIdentifierText(node.object, context)}.${getNodeIdentifierText(
+          node.property,
+          context
+        )}`
+      : isThisExpression(node)
+      ? "this"
+      : isUnaryExpression(node)
+      ? getNodeIdentifierText(node.argument, context)
+      : isExpressionStatement(node)
+      ? context.getSourceCode().getText(node)
+      : isTSArrayType(node) ||
+        isTSIndexSignature(node) ||
+        isTSTupleType(node) ||
+        isTSTypeAnnotation(node) ||
+        isTSTypeLiteral(node) ||
+        isTSTypeReference(node)
+      ? getNodeIdentifierText(node.parent, context)
+      : undefined
     : undefined;
 }
 
diff --git a/src/rules/prefer-tacit.ts b/src/rules/prefer-tacit.ts
index 7e5acd5cb..acbed68c5 100644
--- a/src/rules/prefer-tacit.ts
+++ b/src/rules/prefer-tacit.ts
@@ -11,6 +11,7 @@ import { createRule, getESTreeNode, getTypeOfNode } from "~/util/rule";
 import {
   isBlockStatement,
   isCallExpression,
+  isDefined,
   isFunctionLike,
   isIdentifier,
   isReturnStatement,
@@ -101,8 +102,7 @@ function isCallerViolation(
   const declaration = getESTreeNode(tsDeclaration, context);
 
   return (
-    (declaration !== null &&
-      declaration !== undefined &&
+    (isDefined(declaration) &&
       (isFunctionLike(declaration) || isTSFunctionType(declaration)) &&
       declaration.params.length === caller.arguments.length) ||
     // Check for optional params.
diff --git a/src/util/tree.ts b/src/util/tree.ts
index e1460ed60..e5de771d6 100644
--- a/src/util/tree.ts
+++ b/src/util/tree.ts
@@ -4,6 +4,7 @@ import type { TSESTree } from "@typescript-eslint/experimental-utils";
 import {
   isCallExpression,
   isClassLike,
+  isDefined,
   isFunctionExpressionLike,
   isFunctionLike,
   isIdentifier,
@@ -27,9 +28,9 @@ function getAncestorOfType<T extends TSESTree.Node>(
 ): T | null {
   return checker(node, child)
     ? node
-    : node.parent === null || node.parent === undefined
-    ? null
-    : getAncestorOfType(checker, node.parent, node);
+    : isDefined(node.parent)
+    ? getAncestorOfType(checker, node.parent, node)
+    : null;
 }
 
 /**
@@ -57,11 +58,9 @@ export function inClass(node: TSESTree.Node): boolean {
 export function inReadonly(node: TSESTree.Node): boolean {
   // For nested cases, we shouldn't look for any parent, but the immediate parent.
   if (
-    node.parent !== null &&
-    node.parent !== undefined &&
+    isDefined(node.parent) &&
     isTSTypeLiteral(node.parent) &&
-    node.parent.parent !== null &&
-    node.parent.parent !== undefined &&
+    isDefined(node.parent.parent) &&
     isTSTypeAnnotation(node.parent.parent)
   ) {
     return false;
@@ -102,8 +101,7 @@ export function isInReturnType(node: TSESTree.Node): boolean {
   return (
     getAncestorOfType(
       (n): n is TSESTree.Node =>
-        n.parent !== undefined &&
-        n.parent !== null &&
+        isDefined(n.parent) &&
         isFunctionLike(n.parent) &&
         n.parent.returnType === n,
       node
diff --git a/src/util/typeguard.ts b/src/util/typeguard.ts
index aac9a21c5..170d57168 100644
--- a/src/util/typeguard.ts
+++ b/src/util/typeguard.ts
@@ -334,6 +334,10 @@ export function hasKey(
   return Object.prototype.hasOwnProperty.call(node, "key");
 }
 
+export function isDefined<T>(value: T | null | undefined): value is T {
+  return value !== null && value !== undefined;
+}
+
 /*
  * TS types type guards.
  */
diff --git a/yarn.lock b/yarn.lock
index 80d296ba6..a4d579479 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1154,10 +1154,10 @@
   dependencies:
     "@octokit/openapi-types" "^11.2.0"
 
-"@rebeccastevens/eslint-config@^1.1.4":
-  version "1.1.4"
-  resolved "https://registry.yarnpkg.com/@rebeccastevens/eslint-config/-/eslint-config-1.1.4.tgz#97e656678d97fa3941f6aa817b1eda9a92ac6a98"
-  integrity sha512-nEWzZDhuSwYXVQ48Scovcrlw2WaXekEo1xSiZxBqRo3WyXLf2CsyJYi3wq3DoCIsPpBPmzZ8RostREw/vPDedw==
+"@rebeccastevens/eslint-config@^1.1.5":
+  version "1.1.5"
+  resolved "https://registry.yarnpkg.com/@rebeccastevens/eslint-config/-/eslint-config-1.1.5.tgz#9b640323c99f0969f0aa59a3198f5609b3f82e94"
+  integrity sha512-Gd/g6CqCPOuycycsD3RFAaQ84wobusEA+60dyXvODj66nLaqvS78HcCrWbL69Y28XL5SuXJB8ukB/LK52AbE3A==
   dependencies:
     deepmerge-ts "^1.0.1"