Improve failure message & docs for ban-comma-operator rule (palantir#…

…3380) (palantir#3384)

src/rules/banCommaOperatorRule.ts

@@ -21,19 +21,41 @@ import * as ts from "typescript";
 import * as Lint from "../index";
 
 export class Rule extends Lint.Rules.AbstractRule {
-    /* tslint:disable:object-literal-sort-keys */
+    /* tslint:disable:object-literal-sort-keys max-line-length */
     public static metadata: Lint.IRuleMetadata = {
         ruleName: "ban-comma-operator",
-        description: "Bans the comma operator.",
+        description: "Disallows the comma operator to be used.",
+        descriptionDetails: "[Read more about the comma operator here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Comma_Operator).",
+        rationale: Lint.Utils.dedent`
+            Using the comma operator can create a potential for many non-obvious bugs or lead to misunderstanding of code.
+
+            ### Examples
+            \`\`\`
+            foo((bar, baz)); // evaluates to 'foo(baz)' because of the extra parens - confusing and not obvious
+            \`\`\`
+
+            \`\`\`
+            switch (foo) {
+                case 1, 2: // equals 'case 2' - probably intended 'case 1: case2:'
+                    return true;
+                case 3:
+                    return false;
+            }
+            \`\`\`
+
+            \`\`\`
+            let x = (y = 1, z = 2); // x is equal to 2 - this may not be immediately obvious.
+            \`\`\`
+        `,
         options: null,
         optionsDescription: "",
         optionExamples: [true],
         type: "typescript",
         typescriptOnly: true,
     };
-    /* tslint:enable:object-literal-sort-keys */
+    /* tslint:enable:object-literal-sort-keys max-line-length */
 
-    public static FAILURE_STRING = "Don't use the comma operator.";
+    public static FAILURE_STRING = "Do not use comma operator here because it can be easily misunderstood or lead to unintended bugs.";
 
     public apply(sourceFile: ts.SourceFile): Lint.RuleFailure[] {
         return this.applyWithFunction(sourceFile, walk);

test/rules/ban-comma-operator/test.ts.lint

@@ -1,6 +1,20 @@
 let x = (y = 1, z = 2);
-         ~~~~~~~~~~~~   [Don't use the comma operator.]
+         ~~~~~~~~~~~~   [0]
 
 // Error prone: forgot to add parens around arguments.
 (x, y => x + y)(a, b);
- ~~~~~~~~~~~~~   [Don't use the comma operator.]
+ ~~~~~~~~~~~~~   [0]
+
+foo((bar, baz));
+     ~~~~~~~~    [0]
+
+switch (blah) {
+    case 1, 2: // equals `case 2` - probably intended `case 1: case2:`
+         ~~~~    [0]
+        return true;
+    case 3:
+        return false;
+}
+
+
+[0]: Do not use comma operator here because it can be easily misunderstood or lead to unintended bugs.