fix(eslint-plugin): [prefer-reduce-type-parameter] handle already existing type params (#3706)

Author: yeonjuan

packages/eslint-plugin/src/rules/prefer-reduce-type-parameter.ts

@@ -77,22 +77,31 @@ export default util.createRule({
           context.report({
             messageId: 'preferTypeParameter',
             node: secondArg,
-            fix: fixer => [
-              fixer.removeRange([
-                secondArg.range[0],
-                secondArg.expression.range[0],
-              ]),
-              fixer.removeRange([
-                secondArg.expression.range[1],
-                secondArg.range[1],
-              ]),
-              fixer.insertTextAfter(
-                callee,
-                `<${context
-                  .getSourceCode()
-                  .getText(secondArg.typeAnnotation)}>`,
-              ),
-            ],
+            fix: fixer => {
+              const fixes = [
+                fixer.removeRange([
+                  secondArg.range[0],
+                  secondArg.expression.range[0],
+                ]),
+                fixer.removeRange([
+                  secondArg.expression.range[1],
+                  secondArg.range[1],
+                ]),
+              ];
+
+              if (!callee.parent.typeParameters) {
+                fixes.push(
+                  fixer.insertTextAfter(
+                    callee,
+                    `<${context
+                      .getSourceCode()
+                      .getText(secondArg.typeAnnotation)}>`,
+                  ),
+                );
+              }
+
+              return fixes;
+            },
           });
 
           return;

packages/eslint-plugin/tests/rules/prefer-reduce-type-parameter.test.ts

@@ -39,6 +39,23 @@ ruleTester.run('prefer-reduce-type-parameter', rule, {
     '[1, 2, 3]?.reduce<number[]>((a, s) => a.concat(s * 2), []);',
   ],
   invalid: [
+    {
+      code: `
+declare const arr: string[];
+arr.reduce<string>(acc => acc, arr.shift() as string);
+      `,
+      output: `
+declare const arr: string[];
+arr.reduce<string>(acc => acc, arr.shift());
+      `,
+      errors: [
+        {
+          messageId: 'preferTypeParameter',
+          column: 32,
+          line: 3,
+        },
+      ],
+    },
     {
       code: '[1, 2, 3].reduce((a, s) => a.concat(s * 2), [] as number[]);',
       output: '[1, 2, 3].reduce<number[]>((a, s) => a.concat(s * 2), []);',