Teach no-array-index-key about reduce and reduceRight

As @ljharb pointed out, this rule should spot array indexes when using
reduce. Since the index is the third argument to the reduce callback, I
needed to generalize some of my logic a bit to make this configurable
for each type of function.

Author: lencioni

docs/rules/no-array-index-key.md

@@ -38,6 +38,14 @@ things.every((thing, index) => {
 things.findIndex((thing, index) => {
   otherThings.push(<Hello key={index} />);
 });
+
+things.reduce((collection, thing, index) => (
+  collection.concat(<Hello key={index} />)
+), []);
+
+things.reduceRight((collection, thing, index) => (
+  collection.concat(<Hello key={index} />)
+), []);
 ```
 
 The following patterns are not considered warnings:
@@ -70,4 +78,12 @@ things.every((thing) => {
 things.findIndex((thing) => {
   otherThings.push(<Hello key={thing.id} />);
 });
+
+things.reduce((collection, thing) => (
+  collection.concat(<Hello key={thing.id} />)
+), []);
+
+things.reduceRight((collection, thing) => (
+  collection.concat(<Hello key={thing.id} />)
+), []);
 ```

lib/rules/no-array-index-key.js

@@ -24,15 +24,17 @@ module.exports = {
     // Public
     // --------------------------------------------------------------------------
     var indexParamNames = [];
-    var iteratorFunctionNames = [
-      'every',
-      'filter',
-      'find',
-      'findIndex',
-      'forEach',
-      'map',
-      'some'
-    ];
+    var iteratorFunctionsToIndexParamPosition = {
+      every: 1,
+      filter: 1,
+      find: 1,
+      findIndex: 1,
+      forEach: 1,
+      map: 1,
+      reduce: 2,
+      reduceRight: 2,
+      some: 1
+    };
     var ERROR_MESSAGE = 'Do not use Array index in keys';
 
     function isArrayIndex(node) {
@@ -48,7 +50,7 @@ module.exports = {
       if (callee.property.type !== 'Identifier') {
         return null;
       }
-      if (iteratorFunctionNames.indexOf(callee.property.name) === -1) {
+      if (!iteratorFunctionsToIndexParamPosition.hasOwnProperty(callee.property.name)) {
         return null;
       }
 
@@ -66,11 +68,13 @@ module.exports = {
       }
 
       var params = firstArg.params;
-      if (params.length < 2) {
+
+      var indexParamPosition = iteratorFunctionsToIndexParamPosition[callee.property.name];
+      if (params.length < indexParamPosition + 1) {
         return null;
       }
 
-      return params[1].name;
+      return params[indexParamPosition].name;
     }
 
     function getIdentifiersFromBinaryExpression(side) {

tests/lib/rules/no-array-index-key.js

@@ -71,6 +71,26 @@ ruleTester.run('no-array-index-key', rule, {
       ].join('\n'),
       errors: [{message: 'Do not use Array index in keys'}],
       parserOptions: parserOptions
+    },
+
+    {
+      code: 'foo.reduce((a, b) => a.concat(<Foo key={b.id} />), [])',
+      parserOptions: parserOptions
+    },
+
+    {
+      code: 'foo.reduce((a, b, i) => a.concat(<Foo key={b.id} />), [])',
+      parserOptions: parserOptions
+    },
+
+    {
+      code: 'foo.reduceRight((a, b) => a.concat(<Foo key={b.id} />), [])',
+      parserOptions: parserOptions
+    },
+
+    {
+      code: 'foo.reduceRight((a, b, i) => a.concat(<Foo key={b.id} />), [])',
+      parserOptions: parserOptions
     }
   ],
 
@@ -159,6 +179,18 @@ ruleTester.run('no-array-index-key', rule, {
       parserOptions: parserOptions
     },
 
+    {
+      code: 'foo.reduce((a, b, i) => a.concat(<Foo key={i} />), [])',
+      errors: [{message: 'Do not use Array index in keys'}],
+      parserOptions: parserOptions
+    },
+
+    {
+      code: 'foo.reduceRight((a, b, i) => a.concat(<Foo key={i} />), [])',
+      errors: [{message: 'Do not use Array index in keys'}],
+      parserOptions: parserOptions
+    },
+
     {
       code: 'foo.map((bar, i) => React.createElement(\'Foo\', { key: i }))',
       errors: [{message: 'Do not use Array index in keys'}],