fix(eslint-plugin): [no-floating-promises] finally should be transparent to unhandled promises (#7092)

* finally is transparent to rejection

* Unwrap object

* split up test cases and put ignoreVoid: false on some

* remove duplicated line

Author: kirkwaiblinger

packages/eslint-plugin/src/rules/no-floating-promises.ts

@@ -233,8 +233,8 @@ export default util.createRule<Options, MessageId>({
       }
 
       if (node.type === AST_NODE_TYPES.CallExpression) {
-        // If the outer expression is a call, it must be either a `.then()` or
-        // `.catch()` that handles the promise.
+        // If the outer expression is a call, a `.catch()` or `.then()` with
+        // rejection handler handles the promise.
 
         const catchRejectionHandler = getRejectionHandlerFromCatchCall(node);
         if (catchRejectionHandler) {
@@ -254,10 +254,14 @@ export default util.createRule<Options, MessageId>({
           }
         }
 
-        if (isPromiseFinallyCallWithHandler(node)) {
-          return { isUnhandled: false };
+        // `x.finally()` is transparent to resolution of the promise, so check `x`.
+        // ("object" in this context is the `x` in `x.finally()`)
+        const promiseFinallyObject = getObjectFromFinallyCall(node);
+        if (promiseFinallyObject) {
+          return isUnhandledPromise(checker, promiseFinallyObject);
         }
 
+        // All other cases are unhandled.
         return { isUnhandled: true };
       } else if (node.type === AST_NODE_TYPES.ConditionalExpression) {
         // We must be getting the promise-like value from one of the branches of the
@@ -381,13 +385,12 @@ function getRejectionHandlerFromThenCall(
   }
 }
 
-function isPromiseFinallyCallWithHandler(
+function getObjectFromFinallyCall(
   expression: TSESTree.CallExpression,
-): boolean {
-  return (
-    expression.callee.type === AST_NODE_TYPES.MemberExpression &&
+): TSESTree.Expression | undefined {
+  return expression.callee.type === AST_NODE_TYPES.MemberExpression &&
     expression.callee.property.type === AST_NODE_TYPES.Identifier &&
-    expression.callee.property.name === 'finally' &&
-    expression.arguments.length >= 1
-  );
+    expression.callee.property.name === 'finally'
+    ? expression.callee.object
+    : undefined;
 }

packages/eslint-plugin/tests/rules/no-floating-promises.test.ts

@@ -31,7 +31,6 @@ async function test() {
     .catch(() => {})
     .finally(() => {});
   Promise.resolve('value').catch(() => {});
-  Promise.resolve('value').finally(() => {});
   return Promise.resolve('value');
 }
     `,
@@ -58,7 +57,6 @@ async function test() {
     .catch(() => {})
     .finally(() => {});
   Promise.reject(new Error('message')).catch(() => {});
-  Promise.reject(new Error('message')).finally(() => {});
   return Promise.reject(new Error('message'));
 }
     `,
@@ -77,7 +75,6 @@ async function test() {
     .catch(() => {})
     .finally(() => {});
   (async () => true)().catch(() => {});
-  (async () => true)().finally(() => {});
   return (async () => true)();
 }
     `,
@@ -97,7 +94,6 @@ async function test() {
     .catch(() => {})
     .finally(() => {});
   returnsPromise().catch(() => {});
-  returnsPromise().finally(() => {});
   return returnsPromise();
 }
     `,
@@ -106,7 +102,6 @@ async function test() {
   const x = Promise.resolve();
   const y = x.then(() => {});
   y.catch(() => {});
-  y.finally(() => {});
 }
     `,
     `
@@ -117,7 +112,6 @@ async function test() {
     `
 async function test() {
   Promise.resolve().catch(() => {}), 123;
-  Promise.resolve().finally(() => {}), 123;
   123,
     Promise.resolve().then(
       () => {},
@@ -160,7 +154,6 @@ async function test() {
     .catch(() => {})
     .finally(() => {});
   promiseValue.catch(() => {});
-  promiseValue.finally(() => {});
   return promiseValue;
 }
     `,
@@ -193,12 +186,7 @@ async function test() {
     () => {},
   );
   promiseIntersection.then(() => {}).catch(() => {});
-  promiseIntersection
-    .then(() => {})
-    .catch(() => {})
-    .finally(() => {});
   promiseIntersection.catch(() => {});
-  promiseIntersection.finally(() => {});
   return promiseIntersection;
 }
     `,
@@ -218,7 +206,6 @@ async function test() {
     .catch(() => {})
     .finally(() => {});
   canThen.catch(() => {});
-  canThen.finally(() => {});
   return canThen;
 }
     `,
@@ -315,7 +302,6 @@ async function test() {
     .catch(() => {})
     .finally(() => {});
   promise.catch(() => {});
-  promise.finally(() => {});
   return promise;
 }
     `,
@@ -333,7 +319,6 @@ async function test() {
     ?.then(() => {})
     ?.catch(() => {});
   returnsPromise()?.catch(() => {});
-  returnsPromise()?.finally(() => {});
   return returnsPromise();
 }
     `,
@@ -465,6 +450,31 @@ Promise.reject().catch(definitelyCallable);
       `,
       options: [{ ignoreVoid: false }],
     },
+    {
+      code: `
+Promise.reject()
+  .catch(() => {})
+  .finally(() => {});
+      `,
+    },
+    {
+      code: `
+Promise.reject()
+  .catch(() => {})
+  .finally(() => {})
+  .finally(() => {});
+      `,
+      options: [{ ignoreVoid: false }],
+    },
+    {
+      code: `
+Promise.reject()
+  .catch(() => {})
+  .finally(() => {})
+  .finally(() => {})
+  .finally(() => {});
+      `,
+    },
   ],
 
   invalid: [
@@ -612,7 +622,6 @@ async function test() {
   (async () => true)();
   (async () => true)().then(() => {});
   (async () => true)().catch();
-  (async () => true)().finally();
 }
       `,
       errors: [
@@ -628,10 +637,6 @@ async function test() {
           line: 5,
           messageId: 'floatingVoid',
         },
-        {
-          line: 6,
-          messageId: 'floatingVoid',
-        },
       ],
     },
     {
@@ -940,7 +945,6 @@ async function test() {
   promiseIntersection;
   promiseIntersection.then(() => {});
   promiseIntersection.catch();
-  promiseIntersection.finally();
 }
       `,
       errors: [
@@ -956,10 +960,6 @@ async function test() {
           line: 7,
           messageId: 'floatingVoid',
         },
-        {
-          line: 8,
-          messageId: 'floatingVoid',
-        },
       ],
     },
     {
@@ -1587,5 +1587,69 @@ Promise.reject() || 3;
         },
       ],
     },
+    {
+      code: `
+Promise.reject().finally(() => {});
+      `,
+      errors: [{ line: 2, messageId: 'floatingVoid' }],
+    },
+    {
+      code: `
+Promise.reject()
+  .finally(() => {})
+  .finally(() => {});
+      `,
+      options: [{ ignoreVoid: false }],
+      errors: [{ line: 2, messageId: 'floating' }],
+    },
+    {
+      code: `
+Promise.reject()
+  .finally(() => {})
+  .finally(() => {})
+  .finally(() => {});
+      `,
+      errors: [{ line: 2, messageId: 'floatingVoid' }],
+    },
+    {
+      code: `
+Promise.reject()
+  .then(() => {})
+  .finally(() => {});
+      `,
+      errors: [{ line: 2, messageId: 'floatingVoid' }],
+    },
+    {
+      code: `
+declare const returnsPromise: () => Promise<void> | null;
+returnsPromise()?.finally(() => {});
+      `,
+      errors: [{ line: 3, messageId: 'floatingVoid' }],
+    },
+    {
+      code: `
+const promiseIntersection: Promise<number> & number;
+promiseIntersection.finally(() => {});
+      `,
+      errors: [{ line: 3, messageId: 'floatingVoid' }],
+    },
+    {
+      code: `
+Promise.resolve().finally(() => {}), 123;
+      `,
+      errors: [{ line: 2, messageId: 'floatingVoid' }],
+    },
+    {
+      code: `
+(async () => true)().finally();
+      `,
+      errors: [{ line: 2, messageId: 'floatingVoid' }],
+    },
+    {
+      code: `
+Promise.reject(new Error('message')).finally(() => {});
+      `,
+      errors: [{ line: 2, messageId: 'floatingVoid' }],
+    },
   ],
 });