Add undef-init rule (with yo eslint:rule) and tests for it

docs/rules/undef-init.md

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+# Always initialize variables during declaration. Set it explicitly to undefined, if required. (undef-init)
+
+Please describe the origin of the rule here.
+
+
+## Rule Details
+
+This rule aims to...
+
+Examples of **incorrect** code for this rule:
+
+```js
+
+// fill me in
+
+```
+
+Examples of **correct** code for this rule:
+
+```js
+
+// fill me in
+
+```
+
+### Options
+
+If there are any options, describe them here. Otherwise, delete this section.
+
+## When Not To Use It
+
+Give a short description of when it would be appropriate to turn off this rule.
+
+## Further Reading
+
+If there are other links that describe the issue this rule addresses, please include them here in a bulleted list.

lib/ruleNameMessageMap.js

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+module.exports = {
+    'undef-init': "Always initialize variables during declaration. Set it explicitly to undefined, if required.",
+}

lib/rules/undef-init.js

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+/**
+ * @fileoverview Always initialize variables during declaration. Set it explicitly to undefined, if required.
+ * @author Abhas Bhattacharya
+ */
+"use strict";
+
+const ruleMessage = require('../ruleNameMessageMap')['undef-init'];
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        docs: {
+            description: ruleMessage,
+            category: "Fill me in",
+            recommended: false
+        },
+        fixable: "code",
+        schema: [
+        ]
+    },
+
+    create: function (context) {
+
+        // variables should be defined here
+        const defaultAllowInForConditon = {
+            ForInStatement: true,
+            ForOfStatement: true,
+        }
+
+        //----------------------------------------------------------------------
+        // Public
+        //----------------------------------------------------------------------
+
+        return {
+            "VariableDeclarator": function (node) {
+
+                if (node.init === null) {
+                    // Parent in always VariableDeclarator, whose parent can be directly one of the for in / for of statements
+                    if (node.parent.parent.type === 'ForInStatement' && defaultAllowInForConditon.ForInStatement) {
+                        return;
+                    }
+                    if (node.parent.parent.type === 'ForOfStatement' && defaultAllowInForConditon.ForOfStatement) {
+                        return;
+                    }
+
+                    context.report({
+                        node:node, 
+                        message:ruleMessage,
+                        fix: function(fixer) {
+                            return fixer.insertTextAfter(node, " = undefined");
+                        }
+                    });
+                }
+            }
+        };
+    }
+};

tests/lib/rules/undef-init.js

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+/**
+ * @fileoverview Always initialize variables during declaration. Set it explicitly to undefined, if required.
+ * @author Abhas Bhattacharya
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+var rule = require("../../../lib/rules/undef-init"),
+    RuleTester = require("eslint").RuleTester,
+    ruleMessage = require("../../../lib/ruleNameMessageMap")['undef-init'];
+
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+var ruleTester = new RuleTester({
+    parserOptions: { ecmaVersion: 7 }
+});
+ruleTester.run("undef-init", rule, {
+
+    valid: [
+        "var a = undefined;",
+        "var a = 123;",
+        "let a = [];",
+        "var a = 1; var b = a;",
+        "for (var a of {}) {a;}",
+        "for (var a in []) {a;}",
+    ],
+
+    invalid: [
+        {
+            code: "var a;",
+            errors: [{
+                message: ruleMessage,
+            }],
+            output: "var a = undefined;",
+        },
+        {
+            code: "let  abc; something_else()",
+            errors: [{
+                message: ruleMessage,
+            }],
+            output: "let  abc = undefined; something_else()"
+        },
+        {
+            code: "let a1, a2, a3;",
+            errors: [
+                {
+                    message: ruleMessage,
+                },
+                {
+                    message: ruleMessage,
+                },
+                {
+                    message: ruleMessage,
+                }
+            ],
+            output: "let a1 = undefined, a2 = undefined, a3 = undefined;",
+        },
+        {
+            code: "var a1, a2= 123, a3;",
+            errors: [
+                {
+                    message: ruleMessage,
+                },
+                {
+                    message: ruleMessage,
+                },
+            ],
+            output: "var a1 = undefined, a2= 123, a3 = undefined;",
+        },
+        {
+            code: "for (a of {}){var c;}",
+            errors: [
+                {
+                    message: ruleMessage,
+                },
+            ],
+            output: "for (a of {}){var c = undefined;}",
+        },
+        {
+            code: "for (var a of {}){var c;}",
+            errors: [
+                {
+                    message: ruleMessage,
+                },
+            ],
+            output: "for (var a of {}){var c = undefined;}",
+        },
+        {
+            code: "for (var a in []){var c;}",
+            errors: [
+                {
+                    message: ruleMessage,
+                },
+            ],
+            output: "for (var a in []){var c = undefined;}",
+        },
+    ]
+});