New: Add init-declarations rule (fixes eslint#2606)

conf/eslint.json

@@ -140,6 +140,7 @@
         "guard-for-in": 0,
         "handle-callback-err": 0,
         "indent": 0,
+        "init-declarations": 0,
         "key-spacing": [2, { "beforeColon": false, "afterColon": true }],
         "lines-around-comment": 0,
         "max-depth": [0, 4],

docs/rules/README.md

@@ -106,6 +106,7 @@ These rules relate to using strict mode.
 
 These rules have to do with variable declarations.
 
+* [init-declarations](init-declarations.md) - enforce or disallow variable initializations at definition (off by default)
 * [no-catch-shadow](no-catch-shadow.md) - disallow the catch clause parameter name being the same as a variable in the outer scope (off by default in the node environment)
 * [no-delete-var](no-delete-var.md) - disallow deletion of variables
 * [no-label-var](no-label-var.md) - disallow labels that share a name with a variable

docs/rules/init-declarations.md

@@ -0,0 +1,54 @@
+# Enforce/Disallow Variable Initializations (init-declarations)
+
+In JavaScript, it's possible to redeclare the same variable name using `var`. This can lead to confusion as to where the variable is actually declared and initialized.
+
+## Rule Details
+
+This rule is aimed at enforcing or eliminating variable initializations during declaration. For example, in the following code, `foo` is initialized during declaration, while `bar` is not.
+
+```js
+var foo = 1;
+var bar;
+
+bar = 2;
+```
+
+This rule aims to bring consistency to variable initializations and declarations.
+
+### Options
+
+This rule is configured by passing in the string `"always"` (the default) to enforce initialization at declaration, or `"never"` to disallow initialization during declaration. This rule applies to `var`, `let`, and `const`, however `"never"` is ignored for `const` variables.
+
+You can configure the rule as follows:
+
+```javascript
+{
+    // (default) All variables must be initialized at declaration
+    "init-declarations": [2, "always"],
+
+    // Variables must not be initialized at declaration
+    "init-declarations": [2, "always"]
+}
+```
+
+When configured with `"always"` (the default), the following patterns are considered warnings:
+
+```js
+function foo() {
+    var bar;
+    let baz;
+    const qux;
+}
+```
+
+The following patterns are not considered warnings:
+
+```js
+function foo() {
+    var bar = 1;
+    let baz = 2;
+    const qux = 3;
+}
+```
+
+When configured with `"never"`, the previous conditions are reversed.

lib/rules/init-declarations.js

@@ -0,0 +1,48 @@
+/**
+ * @fileoverview A rule to control the style of variable initializations.
+ * @author Colin Ihrig
+ * @copyright 2015 Colin Ihrig. All rights reserved.
+ */
+
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = function(context) {
+
+    var MODE_ALWAYS = "always",
+        MODE_NEVER = "never";
+
+    var mode = context.options[0] || MODE_ALWAYS;
+
+    //--------------------------------------------------------------------------
+    // Public API
+    //--------------------------------------------------------------------------
+
+    return {
+        "VariableDeclaration": function(node) {
+
+            var kind = node.kind,
+                declarations = node.declarations;
+
+            for (var i = 0; i < declarations.length; ++i) {
+                var initialized = declarations[i].init !== null;
+
+                if (mode === MODE_ALWAYS && !initialized) {
+                    context.report(node, "Variables should be initialized on declaration.");
+                } else if (mode === MODE_NEVER && kind !== "const" && initialized) {
+                    context.report(node, "Variables should not be initialized on declaration.");
+                }
+            }
+        }
+    };
+
+};
+
+module.exports.schema = [
+    {
+        "enum": ["always", "never"]
+    }
+];