Adding "allow-declarations" to only-arrow-functions (palantir#1452)

src/rules/onlyArrowFunctionsRule.ts

@@ -19,15 +19,29 @@ import * as ts from "typescript";
 
 import * as Lint from "../lint";
 
+const OPTION_ALLOW_DECLARATIONS = "allow-declarations";
+
 export class Rule extends Lint.Rules.AbstractRule {
     /* tslint:disable:object-literal-sort-keys */
     public static metadata: Lint.IRuleMetadata = {
         ruleName: "only-arrow-functions",
         description: "Disallows traditional (non-arrow) function expressions.",
         rationale: "Traditional functions don't bind lexical scope, which can lead to unexpected behavior when accessing 'this'.",
-        optionsDescription: "Not configurable.",
-        options: null,
-        optionExamples: ["true"],
+        optionsDescription: Lint.Utils.dedent`
+            One argument may be optionally provided:
+
+            * \`"${OPTION_ALLOW_DECLARATIONS}"\` allows standalone function declarations.
+        `,
+        options: {
+            type: "array",
+            items: {
+                type: "string",
+                enum: [OPTION_ALLOW_DECLARATIONS],
+            },
+            minLength: 0,
+            maxLength: 1,
+        },
+        optionExamples: ["true", `[true, "${OPTION_ALLOW_DECLARATIONS}"]`],
         type: "typescript",
     };
     /* tslint:enable:object-literal-sort-keys */
@@ -41,7 +55,9 @@ export class Rule extends Lint.Rules.AbstractRule {
 
 class OnlyArrowFunctionsWalker extends Lint.RuleWalker {
     public visitFunctionDeclaration(node: ts.FunctionDeclaration) {
-        this.addFailure(this.createFailure(node.getStart(), "function".length, Rule.FAILURE_STRING));
+        if (!this.hasOption(OPTION_ALLOW_DECLARATIONS)) {
+            this.addFailure(this.createFailure(node.getStart(), "function".length, Rule.FAILURE_STRING));
+        }
         super.visitFunctionDeclaration(node);
     }
 

test/rules/only-arrow-functions/allow-declarations/test.ts.lint

@@ -0,0 +1,20 @@
+function foo(a: any) : any {
+    return;
+}
+
+let a = () => {};
+let b = function () {};
+        ~~~~~~~~                 [0]
+
+function c() {}
+
+function () {
+    // ...
+}
+
+((func) => func())(function e(): void {});
+                   ~~~~~~~~                   [0]
+
+((func) => func())(() => {});
+
+[0]: non-arrow functions are forbidden

test/rules/only-arrow-functions/allow-declarations/tslint.json

@@ -0,0 +1,5 @@
+{
+  "rules": {
+    "only-arrow-functions": [true, "allow-declarations"]
+  }
+}