fix(eslint-plugin): [no-unnecessary-condition] handle void (typescript-eslint#5766)

yeonjuan · web-flow · commit ac8f06b68dca · 2022-10-07T12:33:29.000-04:00
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts b/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts
@@ -330,6 +330,7 @@ export default createRule<Options, MessageId>({
       if (isStrictNullChecks) {
         const UNDEFINED = ts.TypeFlags.Undefined;
         const NULL = ts.TypeFlags.Null;
+        const VOID = ts.TypeFlags.Void;
         const isComparable = (type: ts.Type, flag: ts.TypeFlags): boolean => {
           // Allow comparison to `any`, `unknown` or a naked type parameter.
           flag |=
@@ -339,17 +340,17 @@ export default createRule<Options, MessageId>({
 
           // Allow loose comparison to nullish values.
           if (node.operator === '==' || node.operator === '!=') {
-            flag |= NULL | UNDEFINED;
+            flag |= NULL | UNDEFINED | VOID;
           }
 
           return isTypeFlagSet(type, flag);
         };
 
         if (
           (leftType.flags === UNDEFINED &&
-            !isComparable(rightType, UNDEFINED)) ||
+            !isComparable(rightType, UNDEFINED | VOID)) ||
           (rightType.flags === UNDEFINED &&
-            !isComparable(leftType, UNDEFINED)) ||
+            !isComparable(leftType, UNDEFINED | VOID)) ||
           (leftType.flags === NULL && !isComparable(rightType, NULL)) ||
           (rightType.flags === NULL && !isComparable(leftType, NULL))
         ) {
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts
@@ -45,7 +45,7 @@ const unnecessaryConditionTest = (
   errors: [ruleError(4, 12, messageId)],
 });
 
-ruleTester.run('no-unnecessary-conditionals', rule, {
+ruleTester.run('no-unnecessary-condition', rule, {
   valid: [
     `
 declare const b1: boolean;
@@ -67,6 +67,11 @@ for (let i = 0; b1 && b2; i++) {
 }
 const t1 = b1 && b2 ? 'yes' : 'no';
 for (;;) {}
+    `,
+    `
+declare function foo(): number | void;
+const result1 = foo() === undefined;
+const result2 = foo() == null;
     `,
     necessaryConditionTest('false | 5'), // Truthy literal and falsy literal
     necessaryConditionTest('boolean | "foo"'), // boolean and truthy literal
