typescript-eslint · bradzacher · Apr 13, 2020 · Mar 8, 2020 · Mar 8, 2020 · Mar 8, 2020
diff --git a/packages/eslint-plugin/README.md b/packages/eslint-plugin/README.md
@@ -149,6 +149,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
 | [`@typescript-eslint/prefer-optional-chain`](./docs/rules/prefer-optional-chain.md)                                   | Prefer using concise optional chain expressions instead of chained logical ands                                         |                    | :wrench: |                   |
 | [`@typescript-eslint/prefer-readonly`](./docs/rules/prefer-readonly.md)                                               | Requires that private members are marked as `readonly` if they're never modified outside of the constructor             |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/prefer-readonly-parameter-types`](./docs/rules/prefer-readonly-parameter-types.md)               | Requires that function parameters are typed as readonly to prevent accidental mutation of inputs                        |                    |          | :thought_balloon: |
+| [`@typescript-eslint/prefer-reduce-type-parameter`](./docs/rules/prefer-reduce-type-parameter.md)                     | Prefer using type parameter when calling `Array#reduce` instead of casting                                              |                    | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/prefer-regexp-exec`](./docs/rules/prefer-regexp-exec.md)                                         | Enforce that `RegExp#exec` is used instead of `String#match` if no global flag is provided                              | :heavy_check_mark: |          | :thought_balloon: |
 | [`@typescript-eslint/prefer-string-starts-ends-with`](./docs/rules/prefer-string-starts-ends-with.md)                 | Enforce the use of `String#startsWith` and `String#endsWith` instead of other equivalent methods of checking substrings | :heavy_check_mark: | :wrench: | :thought_balloon: |
 | [`@typescript-eslint/promise-function-async`](./docs/rules/promise-function-async.md)                                 | Requires any function or method that returns a Promise to be marked async                                               |                    |          | :thought_balloon: |

diff --git a/packages/eslint-plugin/docs/rules/prefer-reduce-type-parameter.md b/packages/eslint-plugin/docs/rules/prefer-reduce-type-parameter.md
@@ -0,0 +1,54 @@
+# Prefer using type parameter when calling `Array#reduce` instead of casting (`prefer-reduce-type-parameter`)
+
+It's common to call `Array#reduce` with a generic type, such as an array or object, as the initial value.
+Since these values are empty, their types are not usable:
+
+- `[]` has type `never[]`, which can't have items pushed into it as nothing is type `never`
+- `{}` has type `{}`, which doesn't have an index signature and so can't have properties added to it
+
+A common solution to this problem is to cast the initial value. While this will work, it's not the most optimal
+solution as casting has subtle effects on the underlying types that can allow bugs to slip in.
+
+A better (and lesser known) solution is to pass the type in as a generic parameter to `Array#reduce` explicitly.
+This means that TypeScript doesn't have to try to infer the type, and avoids the common pitfalls that come with casting.
+
+## Rule Details
+
+This rule looks for calls to `Array#reduce`, and warns if an initial value is being passed & casted,
+suggesting instead to pass the cast type to `Array#reduce` as its generic parameter.
+
+Examples of **incorrect** code for this rule:
+
+```ts
+[1, 2, 3].reduce((arr, num) => arr.concat(num * 2), [] as number[]);
+
+['a', 'b'].reduce(
+  (accum, name) => ({
+    ...accum,
+    [name]: true,
+  }),
+  {} as Record<string, boolean>,
+);
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+[1, 2, 3].reduce<number[]>((arr, num) => arr.concat(num * 2), []);
+
+['a', 'b'].reduce<Record<string, boolean>>(
+  (accum, name) => ({
+    ...accum,
+    [name]: true,
+  }),
+  {},
+);
+```
+
+## Options
+
+There are no options.
+
+## When Not To Use It
+
+If you don't want to use typechecking in your linting, you can't use this rule.
diff --git a/packages/eslint-plugin/src/configs/all.json b/packages/eslint-plugin/src/configs/all.json
@@ -86,6 +86,7 @@
     "@typescript-eslint/prefer-optional-chain": "error",
     "@typescript-eslint/prefer-readonly": "error",
     "@typescript-eslint/prefer-readonly-parameter-types": "error",
+    "@typescript-eslint/prefer-reduce-type-parameter": "error",
     "@typescript-eslint/prefer-regexp-exec": "error",
     "@typescript-eslint/prefer-string-starts-ends-with": "error",
     "@typescript-eslint/promise-function-async": "error",

diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -75,6 +75,7 @@ import preferNullishCoalescing from './prefer-nullish-coalescing';
 import preferOptionalChain from './prefer-optional-chain';
 import preferReadonly from './prefer-readonly';
 import preferReadonlyParameterTypes from './prefer-readonly-parameter-types';
+import preferReduceTypeParameter from './prefer-reduce-type-parameter';
 import preferRegexpExec from './prefer-regexp-exec';
 import preferStringStartsEndsWith from './prefer-string-starts-ends-with';
 import promiseFunctionAsync from './promise-function-async';
@@ -172,6 +173,7 @@ export default {
   'prefer-optional-chain': preferOptionalChain,
   'prefer-readonly-parameter-types': preferReadonlyParameterTypes,
   'prefer-readonly': preferReadonly,
+  'prefer-reduce-type-parameter': preferReduceTypeParameter,
   'prefer-regexp-exec': preferRegexpExec,
   'prefer-string-starts-ends-with': preferStringStartsEndsWith,
   'promise-function-async': promiseFunctionAsync,

diff --git a/packages/eslint-plugin/src/rules/prefer-reduce-type-parameter.ts b/packages/eslint-plugin/src/rules/prefer-reduce-type-parameter.ts
@@ -0,0 +1,104 @@
+import {
+  AST_NODE_TYPES,
+  TSESTree,
+} from '@typescript-eslint/experimental-utils';
+import * as util from '../util';
+
+type MemberExpressionWithCallExpressionParent = (
+  | TSESTree.MemberExpression
+  | TSESTree.OptionalMemberExpression
+) & { parent: TSESTree.CallExpression | TSESTree.OptionalCallExpression };
+
+const getMemberExpressionName = (
+  member: TSESTree.MemberExpression | TSESTree.OptionalMemberExpression,
+): string | null => {
+  if (!member.computed) {
+    return member.property.name;
+  }
+
+  if (
+    member.property.type === AST_NODE_TYPES.Literal &&
+    typeof member.property.value === 'string'
+  ) {
+    return member.property.value;
+  }
+
+  return null;
+};
+
+export default util.createRule({
+  name: 'prefer-reduce-type-parameter',
+  meta: {
+    type: 'problem',
+    docs: {
+      category: 'Best Practices',
+      recommended: false,
+      description:
+        'Prefer using type parameter when calling `Array#reduce` instead of casting',
+      requiresTypeChecking: true,
+    },
+    messages: {
+      preferTypeParameter:
+        'Unnecessary cast: Array#reduce accepts a type parameter for the default value.',
+    },
+    fixable: 'code',
+    schema: [],
+  },
+  defaultOptions: [],
+  create(context) {
+    const service = util.getParserServices(context);
+    const checker = service.program.getTypeChecker();
+
+    return {
+      ':matches(CallExpression, OptionalCallExpression) > :matches(MemberExpression, OptionalMemberExpression).callee'(
+        callee: MemberExpressionWithCallExpressionParent,
+      ): void {
+        if (getMemberExpressionName(callee) !== 'reduce') {
+          return;
+        }
+
+        const [, secondArg] = callee.parent.arguments;
+
+        if (
+          callee.parent.arguments.length < 2 ||
+          !util.isTypeAssertion(secondArg)
+        ) {
+          return;
+        }
+
+        // Get the symbol of the `reduce` method.
+        const tsNode = service.esTreeNodeToTSNodeMap.get(callee.object);
+        const calleeObjType = util.getConstrainedTypeAtLocation(
+          checker,
+          tsNode,
+        );
+
+        // Check the owner type of the `reduce` method.
+        if (checker.isArrayType(calleeObjType)) {
+          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)}>`,
+              ),
+            ],
+          });
+
+          return;
+        }
+      },
+    };
+  },
+});
diff --git a/packages/eslint-plugin/tests/fixtures/class.ts b/packages/eslint-plugin/tests/fixtures/class.ts
@@ -3,3 +3,8 @@ export class Error {}
 
 // used by unbound-method test case to test imports
 export const console = { log() {} };
+
+// used by prefer-reduce-type-parameter to test native vs userland check
+export class Reducable {
+  reduce() {}
+}
diff --git a/packages/eslint-plugin/tests/rules/prefer-reduce-type-parameter.test.ts b/packages/eslint-plugin/tests/rules/prefer-reduce-type-parameter.test.ts
@@ -0,0 +1,191 @@
+import rule from '../../src/rules/prefer-reduce-type-parameter';
+import { RuleTester, getFixturesRootDir } from '../RuleTester';
+
+const rootPath = getFixturesRootDir();
+
+const ruleTester = new RuleTester({
+  parser: '@typescript-eslint/parser',
+  parserOptions: {
+    sourceType: 'module',
+    tsconfigRootDir: rootPath,
+    project: './tsconfig.json',
+  },
+});
+
+ruleTester.run('prefer-reduce-type-parameter', rule, {
+  valid: [
+    `
+      new (class Mine {
+        reduce() {}
+      })().reduce(() => {}, 1 as any);
+    `,
+    `
+      class Mine {
+        reduce() {}
+      }
+
+      new Mine().reduce(() => {}, 1 as any);
+    `,
+    `
+      import { Reducable } from './class';
+
+      new Reducable().reduce(() => {}, 1 as any);
+    `,
+    "[1, 2, 3]['reduce']((sum, num) => sum + num, 0);",
+    '[1, 2, 3][null]((sum, num) => sum + num, 0);',
+    '[1, 2, 3]?.[null]((sum, num) => sum + num, 0);',
+    '[1, 2, 3].reduce((sum, num) => sum + num, 0);',
+    '[1, 2, 3].reduce<number[]>((a, s) => a.concat(s * 2), []);',
+    '[1, 2, 3]?.reduce<number[]>((a, s) => a.concat(s * 2), []);',
+  ],
+  invalid: [
+    {
+      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), []);',
+      errors: [
+        {
+          messageId: 'preferTypeParameter',
+          column: 45,
+          line: 1,
+        },
+      ],
+    },
+    {
+      code: '[1, 2, 3].reduce((a, s) => a.concat(s * 2), <number[]>[]);',
+      output: '[1, 2, 3].reduce<number[]>((a, s) => a.concat(s * 2), []);',
+      errors: [
+        {
+          messageId: 'preferTypeParameter',
+          column: 45,
+          line: 1,
+        },
+      ],
+    },
+    {
+      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), []);',
+      errors: [
+        {
+          messageId: 'preferTypeParameter',
+          column: 46,
+          line: 1,
+        },
+      ],
+    },
+    {
+      code: '[1, 2, 3]?.reduce((a, s) => a.concat(s * 2), <number[]>[]);',
+      output: '[1, 2, 3]?.reduce<number[]>((a, s) => a.concat(s * 2), []);',
+      errors: [
+        {
+          messageId: 'preferTypeParameter',
+          column: 46,
+          line: 1,
+        },
+      ],
+    },
+    {
+      code: `
+const names = ['a', 'b', 'c'];
+
+names.reduce(
+  (accum, name) => ({
+    ...accum,
+    [name]: true,
+  }),
+  {} as Record<string, boolean>,
+);
+      `,
+      output: `
+const names = ['a', 'b', 'c'];
+
+names.reduce<Record<string, boolean>>(
+  (accum, name) => ({
+    ...accum,
+    [name]: true,
+  }),
+  {},
+);
+      `,
+      errors: [
+        {
+          messageId: 'preferTypeParameter',
+          column: 3,
+          line: 9,
+        },
+      ],
+    },
+    {
+      code: `
+['a', 'b'].reduce(
+  (accum, name) => ({
+    ...accum,
+    [name]: true,
+  }),
+  <Record<string, boolean>>{},
+);
+      `,
+      output: `
+['a', 'b'].reduce<Record<string, boolean>>(
+  (accum, name) => ({
+    ...accum,
+    [name]: true,
+  }),
+  {},
+);
+      `,
+      errors: [
+        {
+          messageId: 'preferTypeParameter',
+          column: 3,
+          line: 7,
+        },
+      ],
+    },
+    {
+      code: `
+['a', 'b']['reduce'](
+  (accum, name) => ({
+    ...accum,
+    [name]: true,
+  }),
+  {} as Record<string, boolean>,
+);
+      `,
+      output: `
+['a', 'b']['reduce']<Record<string, boolean>>(
+  (accum, name) => ({
+    ...accum,
+    [name]: true,
+  }),
+  {},
+);
+      `,
+      errors: [
+        {
+          messageId: 'preferTypeParameter',
+          column: 3,
+          line: 7,
+        },
+      ],
+    },
+    {
+      code: `
+function f<T, U extends T[]>(a: U) {
+  return a.reduce(() => {}, {} as Record<string, boolean>);
+}
+      `,
+      output: `
+function f<T, U extends T[]>(a: U) {
+  return a.reduce<Record<string, boolean>>(() => {}, {});
+}
+      `,
+      errors: [
+        {
+          messageId: 'preferTypeParameter',
+          column: 29,
+          line: 3,
+        },
+      ],
+    },
+  ],
+});