From 1a02bdca0b9acecc9cea00d97cec102867ae8376 Mon Sep 17 00:00:00 2001
From: Jakob Guddas <github@jguddas.de>
Date: Mon, 16 May 2022 09:52:22 +0200
Subject: [PATCH] feat(eslint-plugin): added checking of loose equal ternary
 cases for prefer-nullish-coalescing rule

---
 .../docs/rules/prefer-nullish-coalescing.md   |  4 ++
 .../src/rules/prefer-nullish-coalescing.ts    | 46 ++++++++++++++++++-
 .../rules/prefer-nullish-coalescing.test.ts   | 18 +++++++-
 3 files changed, 65 insertions(+), 3 deletions(-)

diff --git a/packages/eslint-plugin/docs/rules/prefer-nullish-coalescing.md b/packages/eslint-plugin/docs/rules/prefer-nullish-coalescing.md
index 81fa97e6f9e9..c552d684fd7f 100644
--- a/packages/eslint-plugin/docs/rules/prefer-nullish-coalescing.md
+++ b/packages/eslint-plugin/docs/rules/prefer-nullish-coalescing.md
@@ -71,6 +71,8 @@ Incorrect code for `ignoreTernaryTests: false`, and correct code for `ignoreTern
 const foo: any = 'bar';
 foo !== undefined && foo !== null ? foo : 'a string';
 foo === undefined || foo === null ? 'a string' : foo;
+foo == undefined ? 'a string' : foo;
+foo == null ? 'a string' : foo;
 
 const foo: ?string = 'bar';
 foo !== undefined ? foo : 'a string';
@@ -87,6 +89,8 @@ Correct code for `ignoreTernaryTests: false`:
 const foo: any = 'bar';
 foo ?? 'a string';
 foo ?? 'a string';
+foo ?? 'a string';
+foo ?? 'a string';
 
 const foo: ?string = 'bar';
 foo ?? 'a string';
diff --git a/packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts b/packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts
index 1bf25bc487ed..90ce162bf600 100644
--- a/packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts
+++ b/packages/eslint-plugin/src/rules/prefer-nullish-coalescing.ts
@@ -93,10 +93,11 @@ export default util.createRule<Options, MessageIds>({
         if (
           node.consequent.type === AST_NODE_TYPES.Identifier &&
           ((node.test.type === AST_NODE_TYPES.BinaryExpression &&
-            node.test.operator === '!==') ||
+            (node.test.operator === '!==' || node.test.operator === '!=')) ||
             (node.test.type === AST_NODE_TYPES.LogicalExpression &&
               node.test.left.type === AST_NODE_TYPES.BinaryExpression &&
-              node.test.left.operator === '!=='))
+              (node.test.left.operator === '!==' ||
+                node.test.left.operator === '!=')))
         ) {
           identifier = node.consequent;
           alternate = node.alternate;
@@ -110,6 +111,11 @@ export default util.createRule<Options, MessageIds>({
         }
 
         if (
+          isFixableLooseTernary({
+            requiredOperator,
+            identifier,
+            node,
+          }) ||
           isFixableExplicitTernary({
             requiredOperator,
             identifier,
@@ -328,6 +334,42 @@ function isFixableExplicitTernary({
   return true;
 }
 
+function isFixableLooseTernary({
+  node,
+  identifier,
+  requiredOperator,
+}: {
+  requiredOperator: '!==' | '===';
+  identifier: TSESTree.Identifier;
+  node: TSESTree.ConditionalExpression;
+}): boolean {
+  if (node.test.type !== AST_NODE_TYPES.BinaryExpression) {
+    return false;
+  }
+
+  const { left, right, operator } = node.test;
+  if (requiredOperator === '===' && operator !== '==') {
+    return false;
+  }
+  if (requiredOperator === '!==' && operator !== '!=') {
+    return false;
+  }
+
+  const isIdentifier = (
+    i: TSESTree.Expression | TSESTree.PrivateIdentifier,
+  ): boolean =>
+    i.type === AST_NODE_TYPES.Identifier && i.name === identifier.name;
+
+  if (isIdentifier(right) && (isNull(left) || isUndefined(left))) {
+    return true;
+  }
+
+  if (isIdentifier(left) && (isNull(right) || isUndefined(right))) {
+    return true;
+  }
+  return false;
+}
+
 /**
  * This is for cases where we check either undefined or null and fall back to
  * using type information to ensure that our checks are correct.
diff --git a/packages/eslint-plugin/tests/rules/prefer-nullish-coalescing.test.ts b/packages/eslint-plugin/tests/rules/prefer-nullish-coalescing.test.ts
index 94752b2a1bbb..b5094608dab1 100644
--- a/packages/eslint-plugin/tests/rules/prefer-nullish-coalescing.test.ts
+++ b/packages/eslint-plugin/tests/rules/prefer-nullish-coalescing.test.ts
@@ -86,6 +86,14 @@ x ?? 'foo';
       'x === undefined || x === null ? x : y;',
       'x !== undefined || x === null ? x : y;',
       'x !== undefined || x === null ? y : x;',
+      'x == null ? x : y;',
+      'x == undefined ? x : y;',
+      'x != null ? y : x;',
+      'x != undefined ? y : x;',
+      'null == x ? x : y;',
+      'undefined == x ? x : y;',
+      'null != x ? y : x;',
+      'undefined != x ? y : x;',
       `
 declare const x: string | undefined | null;
 x !== undefined ? x : y;
@@ -211,6 +219,14 @@ x ?? 'foo';
       'null !== x && undefined !== x ? x : y;',
       'undefined === x || null === x ? y : x;',
       'null === x || undefined === x ? y : x;',
+      'undefined != x ? x : y;',
+      'null != x ? x : y;',
+      'undefined == x  ? y : x;',
+      'null == x ? y : x;',
+      'x != undefined ? x : y;',
+      'x != null ? x : y;',
+      'x == undefined  ? y : x;',
+      'x == null ? y : x;',
     ].map(code => ({
       code,
       output: null,
@@ -221,7 +237,7 @@ x ?? 'foo';
           line: 1,
           column: 1,
           endLine: 1,
-          endColumn: 38,
+          endColumn: code.length,
           suggestions: [
             {
               messageId: 'suggestNullish' as const,