feat(no-let): add option to allow lets inside of for loop initializers

fix #270

docs/rules/no-let.md

@@ -62,10 +62,44 @@ The default options:
 
 ```ts
 const defaults = {
+  allowInForLoopInit: false,
   allowLocalMutation: false
 }
 ```
 
+### `allowInForLoopInit`
+
+If set, `let`s inside of for a loop initializer are allowed. This does not include for...of or for...in loops.
+
+Examples of **correct** code for this rule:
+
+```js
+/* eslint functional/no-let: ["error", { "allowInForLoopInit": true } ] */
+
+for (let i = 0; i < array.length; i++) {
+}
+```
+
+Examples of **incorrect** code for this rule:
+
+<!-- eslint-skip -->
+
+```js
+/* eslint functional/no-let: "error" */
+
+for (let element of array) {
+}
+```
+
+<!-- eslint-skip -->
+
+```js
+/* eslint functional/no-let: "error" */
+
+for (let [index, element] of array.entries()) {
+}
+```
+
 ### `allowLocalMutation`
 
 See the [allowLocalMutation](./options/allow-local-mutation.md) docs.

src/rules/no-let.ts

@@ -14,20 +14,33 @@ import {
 } from "~/common/ignore-options";
 import type { RuleContext, RuleMetaData, RuleResult } from "~/util/rule";
 import { createRule } from "~/util/rule";
+import { inForLoopInitializer } from "~/util/tree";
 
 // The name of this rule.
 export const name = "no-let" as const;
 
 // The options this rule can take.
-type Options = AllowLocalMutationOption & IgnorePatternOption;
+type Options = AllowLocalMutationOption &
+  IgnorePatternOption & {
+    readonly allowInForLoopInit: boolean;
+  };
 
 // The schema for the rule options.
 const schema: JSONSchema4 = [
-  deepmerge(allowLocalMutationOptionSchema, ignorePatternOptionSchema),
+  deepmerge(allowLocalMutationOptionSchema, ignorePatternOptionSchema, {
+    type: "object",
+    properties: {
+      allowInForLoopInit: {
+        type: "boolean",
+      },
+    },
+    additionalProperties: false,
+  }),
 ];
 
 // The default options for the rule.
 const defaultOptions: Options = {
+  allowInForLoopInit: false,
   allowLocalMutation: false,
 };
 
@@ -57,8 +70,10 @@ function checkVariableDeclaration(
   options: Options
 ): RuleResult<keyof typeof errorMessages, Options> {
   if (
+    node.kind !== "let" ||
     shouldIgnoreLocalMutation(node, context, options) ||
-    shouldIgnorePattern(node, context, options)
+    shouldIgnorePattern(node, context, options) ||
+    (options.allowInForLoopInit && inForLoopInitializer(node))
   ) {
     return {
       context,
@@ -68,7 +83,7 @@ function checkVariableDeclaration(
 
   return {
     context,
-    descriptors: node.kind === "let" ? [{ node, messageId: "generic" }] : [],
+    descriptors: [{ node, messageId: "generic" }],
   };
 }
 

src/util/tree.ts

@@ -5,6 +5,7 @@ import {
   isCallExpression,
   isClassLike,
   isDefined,
+  isForStatement,
   isFunctionExpressionLike,
   isFunctionLike,
   isIdentifier,
@@ -53,6 +54,18 @@ export function inClass(node: TSESTree.Node): boolean {
   return getAncestorOfType(isClassLike, node) !== null;
 }
 
+/**
+ * Test if the given node is in a for loop initializer.
+ */
+export function inForLoopInitializer(node: TSESTree.Node): boolean {
+  return (
+    getAncestorOfType(
+      (n, c): n is TSESTree.ForStatement => isForStatement(n) && n.init === c,
+      node
+    ) !== null
+  );
+}
+
 /**
  * Test if the given node is shallowly inside a `Readonly<{...}>`.
  */

src/util/typeguard.ts

@@ -118,6 +118,12 @@ export function isExpressionStatement(
   return node.type === AST_NODE_TYPES.ExpressionStatement;
 }
 
+export function isForStatement(
+  node: TSESTree.Node
+): node is TSESTree.ForStatement {
+  return node.type === AST_NODE_TYPES.ForStatement;
+}
+
 export function isFunctionDeclaration(
   node: TSESTree.Node
 ): node is TSESTree.FunctionDeclaration {

tests/rules/no-let/es6/valid.ts

@@ -142,6 +142,10 @@ const tests: ReadonlyArray<ValidTestCase> = [
     `,
     optionsSet: [[{ ignorePattern: "Mutable$" }]],
   },
+  {
+    code: `for (let x = 0; x < 1; x++);`,
+    optionsSet: [[{ allowInForLoopInit: true }]],
+  },
 ];
 
 export default tests;