Created no-dangle rule.

Created the no-dangle rule, which warns when it encounters a trailing comma in an object literal. The following will raise warnings:

var foo = {
  bar: "baz",
}

Fixes issue eslint#13

conf/eslint.json

@@ -5,6 +5,7 @@
         "no-caller": 1,
         "no-bitwise": 0,
         "no-console": 1,
+        "no-dangle": 1,
         "no-debugger": 1,
         "no-empty": 1,
         "no-eval": 1,

docs/Rules.md

@@ -7,6 +7,7 @@ Rules in ESLint are divided into several categories to help you better understan
 The following rules point out areas where you might have made mistakes.
 
 * [no-console](no-console.md) - disallow use of `console`
+* [no-dangle](no-dangle.md) - disallow trailing commas in object literals
 * [no-debugger](No-debugger.md) - disallow use of `debugger`
 * [no-empty](No-empty.md) - disallow empty statements
 * [no-unreachable](No-unreachable.md) - disallow unreachable statements after a return, throw, continue, or break statement

docs/no-dangle.md

@@ -0,0 +1,50 @@
+# no dangle
+
+Trailing commas in object literals are valid according to the ECMAScript 5 (and ECMAScript 3!) spec, however IE8 (when not in IE8 document mode) and below will throw an error when it encounters trailing commas in JavaScript.
+
+```js
+var foo = {
+    bar: "baz",
+    qux: "quux",
+};
+```
+
+## Rule Details
+
+This rule is aimed at detecting trailing commas in object literals. As such, it will warn whenever it encounters a trailing comma in an object literal.
+
+The following are considered warnings:
+
+```js
+var foo = {
+    bar: "baz",
+    qux: "quux",
+};
+
+var arr = [1,2,];
+
+foo({
+  bar: "baz",
+  qux: "quux",
+});
+```
+
+The following are okay and will not raise warnings:
+
+```js
+var foo = {
+    bar: "baz",
+    qux: "quux"
+};
+
+var arr = [1,2];
+
+foo({
+  bar: "baz",
+  qux: "quux"
+});
+```
+
+## When Not To Use It
+
+If your code will not be run in IE8 or below (a NodeJS application, for example) and you'd prefer to allow trailing commas, turn this rule off.

lib/rules/no-dangle.js

@@ -0,0 +1,36 @@
+/**
+ * @fileoverview Rule to flag trailing commas in object literals.
+ * @author Ian Christian Myers
+ */
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = function(context) {
+
+     //-------------------------------------------------------------------------
+     // Helpers
+     //-------------------------------------------------------------------------
+
+    function checkForTrailingComma(node) {
+        var tokens = context.getTokens(node),
+            secondToLastToken = tokens[tokens.length - 2];
+
+        // The last token in an object/array literal will always be a closing 
+        // curly, so we check the second to last token for a comma.
+        if (secondToLastToken.value === ",") {
+            context.report(node, "Found trailing comma in object literal.");
+        }
+    }
+
+    //--------------------------------------------------------------------------
+    // Public API
+    //--------------------------------------------------------------------------
+
+    return {
+        "ObjectExpression": checkForTrailingComma,
+        "ArrayExpression": checkForTrailingComma
+    };
+
+};

tests/lib/rules/no-dangle.js

@@ -0,0 +1,106 @@
+/**
+ * @fileoverview Tests for no-dangle rule.
+ * @author Ian Christian Myers
+ */
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+var vows = require("vows"),
+    assert = require("assert"),
+    eslint = require("../../../lib/eslint");
+
+//------------------------------------------------------------------------------
+// Constants
+//------------------------------------------------------------------------------
+
+var RULE_ID = "no-dangle";
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+vows.describe(RULE_ID).addBatch({
+
+    "when evaluating object literal with a dangling comma": {
+
+        topic: "var foo = { bar: \"baz\", }",
+
+        "should report a violation": function(topic) {
+            var config = { rules: {} };
+            config.rules[RULE_ID] = 1;
+
+            var messages = eslint.verify(topic, config);
+
+            assert.equal(messages.length, 1);
+            assert.equal(messages[0].ruleId, RULE_ID);
+            assert.equal(messages[0].message, "Found trailing comma in object literal.");
+            assert.include(messages[0].node.type, "ObjectExpression");
+        }
+    },
+
+    "when evaluating object literal passed to a function with a dangling comma": {
+
+        topic: "foo({ bar: \"baz\", qux: \"quux\", });",
+
+        "should report a violation": function(topic) {
+            var config = { rules: {} };
+            config.rules[RULE_ID] = 1;
+
+            var messages = eslint.verify(topic, config);
+
+            assert.equal(messages.length, 1);
+            assert.equal(messages[0].ruleId, RULE_ID);
+            assert.equal(messages[0].message, "Found trailing comma in object literal.");
+            assert.include(messages[0].node.type, "ObjectExpression");
+        }
+    },
+
+
+    "when evaluating object literal without a dangling comma": {
+
+        topic: "var foo = { bar: \"baz\" }",
+
+        "should not report a violation": function(topic) {
+            var config = { rules: {} };
+            config.rules[RULE_ID] = 1;
+
+            var messages = eslint.verify(topic, config);
+
+            assert.equal(messages.length, 0);
+        }
+    },
+
+    "when evaluating array literal with a dangling comma": {
+
+        topic: "var foo = [ \"baz\", ]",
+
+        "should report a violation": function(topic) {
+            var config = { rules: {} };
+            config.rules[RULE_ID] = 1;
+
+            var messages = eslint.verify(topic, config);
+
+            assert.equal(messages.length, 1);
+            assert.equal(messages[0].ruleId, RULE_ID);
+            assert.equal(messages[0].message, "Found trailing comma in object literal.");
+            assert.include(messages[0].node.type, "ArrayExpression");
+        }
+    },
+
+    "when evaluating array literal without a dangling comma": {
+
+        topic: "var foo = [ \"baz\" ]",
+
+        "should not report a violation": function(topic) {
+            var config = { rules: {} };
+            config.rules[RULE_ID] = 1;
+
+            var messages = eslint.verify(topic, config);
+
+            assert.equal(messages.length, 0);
+        }
+    }
+
+}).export(module);