fix(eslint-plugin): [no-unnecessary-condition] handle comparison of any, unknown and loose comparisons with nullish values (typescript-eslint#2123)

avocadowastaken · web-flow · commit 1ae1d01e5603 · 2020-05-30T14:50:19.000-07:00
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts b/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts
@@ -5,7 +5,6 @@ import {
 } from '@typescript-eslint/experimental-utils';
 import * as ts from 'typescript';
 import {
-  isTypeFlagSet,
   unionTypeParts,
   isFalsyType,
   isBooleanLiteralType,
@@ -14,6 +13,7 @@ import {
   isStrictCompilerOptionEnabled,
 } from 'tsutils';
 import {
+  isTypeFlagSet,
   createRule,
   getParserServices,
   getConstrainedTypeAtLocation,
@@ -23,9 +23,6 @@ import {
   isMemberOrOptionalMemberExpression,
 } from '../util';
 
-const typeContainsFlag = (type: ts.Type, flag: ts.TypeFlags): boolean => {
-  return unionTypeParts(type).some(t => isTypeFlagSet(t, flag));
-};
 // Truthiness utilities
 // #region
 const isTruthyLiteral = (type: ts.Type): boolean =>
@@ -268,13 +265,20 @@ export default createRule<Options, MessageId>({
       if (isStrictCompilerOptionEnabled(compilerOptions, 'strictNullChecks')) {
         const UNDEFINED = ts.TypeFlags.Undefined;
         const NULL = ts.TypeFlags.Null;
+
+        const NULLISH =
+          node.operator === '==' || node.operator === '!='
+            ? NULL | UNDEFINED
+            : NULL;
+
         if (
           (leftType.flags === UNDEFINED &&
-            !typeContainsFlag(rightType, UNDEFINED)) ||
+            !isTypeFlagSet(rightType, UNDEFINED, true)) ||
           (rightType.flags === UNDEFINED &&
-            !typeContainsFlag(leftType, UNDEFINED)) ||
-          (leftType.flags === NULL && !typeContainsFlag(rightType, NULL)) ||
-          (rightType.flags === NULL && !typeContainsFlag(leftType, NULL))
+            !isTypeFlagSet(leftType, UNDEFINED, true)) ||
+          (leftType.flags === NULL &&
+            !isTypeFlagSet(rightType, NULLISH, true)) ||
+          (rightType.flags === NULL && !isTypeFlagSet(leftType, NULLISH, true))
         ) {
           context.report({ node, messageId: 'noOverlapBooleanExpression' });
           return;
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts
@@ -100,13 +100,68 @@ function test<T>(t: T | []) {
     // Boolean expressions
     `
 function test(a: string) {
-  return a === 'a';
+  const t1 = a === 'a';
+  const t2 = 'a' === a;
 }
     `,
     `
 function test(a?: string) {
   const t1 = a === undefined;
-  const t3 = undefined === a;
+  const t2 = undefined === a;
+  const t1 = a !== undefined;
+  const t2 = undefined !== a;
+}
+    `,
+    `
+function test(a: null | string) {
+  const t1 = a === null;
+  const t2 = null === a;
+  const t1 = a !== null;
+  const t2 = null !== a;
+}
+    `,
+    `
+function test(a?: null | string) {
+  const t1 = a == null;
+  const t2 = null == a;
+  const t3 = a != null;
+  const t4 = null != a;
+  const t5 = a == undefined;
+  const t6 = undefined == a;
+  const t7 = a != undefined;
+  const t8 = undefined != a;
+}
+    `,
+    `
+function test(a?: string) {
+  const t1 = a == null;
+  const t2 = null == a;
+  const t3 = a != null;
+  const t4 = null != a;
+}
+    `,
+    `
+function test(a?: null | string) {
+  const t1 = a == null;
+  const t2 = null == a;
+  const t3 = a != null;
+  const t4 = null != a;
+}
+    `,
+    `
+function test(a: any) {
+  const t1 = a == null;
+  const t2 = null == a;
+  const t3 = a != null;
+  const t4 = null != a;
+}
+    `,
+    `
+function test(a: unknown) {
+  const t1 = a == null;
+  const t2 = null == a;
+  const t3 = a != null;
+  const t4 = null != a;
 }
     `,
 
