fix(eslint-plugin): [no-unnecessary-condition] false positive when array predicate returns unknown (typescript-eslint#2772)

ArnaudBarre · web-flow · commit 111c244c3eb1 · 2020-11-16T19:30:20.000-08:00
diff --git a/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts b/packages/eslint-plugin/src/rules/no-unnecessary-condition.ts
@@ -450,6 +450,10 @@ export default createRule<Options, MessageId>({
           // Not a callable function
           return;
         }
+        // Predicate is always necessary if it involves `any` or `unknown`
+        if (returnTypes.some(t => isTypeAnyType(t) || isTypeUnknownType(t))) {
+          return;
+        }
         if (!returnTypes.some(isPossiblyFalsy)) {
           return context.report({
             node: callback,
diff --git a/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts b/packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts
@@ -235,6 +235,14 @@ function length(x: string) {
 function nonEmptyStrings(x: string[]) {
   return x.filter(length);
 }
+
+// filter-like predicate
+function count(
+  list: string[],
+  predicate: (value: string, index: number, array: string[]) => unknown,
+) {
+  return list.filter(predicate).length;
+}
     `,
     // Ignores non-array methods of the same name
     `
