Converted prefer-declare-function from TSLint to ESLint (#538)

* Converted prefer-declare-function from TSLint to ESLint

* Rule applies to all variable declarations, not just exported ones

* Check non-exported consts too, but only in .d.ts files

Co-authored-by: Nathan Shively-Sanders <293473+sandersn@users.noreply.github.com>

packages/dtslint/dtslint.json

@@ -10,7 +10,6 @@
         "no-relative-import-in-test": true,
         "strict-export-declare-modifiers": true,
         "no-single-declare-module": true,
-        "prefer-declare-function": true,
         "unified-signatures": true,
         "void-return": true,
         "npm-naming": true,

packages/dtslint/src/rules/prefer-declare-function.ts

@@ -0,0 +1,37 @@
+import { AST_NODE_TYPES, TSESTree } from "@typescript-eslint/utils";
+
+import { createRule } from "../util";
+
+const rule = createRule({
+  defaultOptions: [],
+  meta: {
+    docs: {
+      description: "Forbids `const x: () => void`.",
+      recommended: "error",
+    },
+    messages: {
+      variableFunction: "Use a function declaration instead of a variable of function type.",
+    },
+    schema: [],
+    type: "problem",
+  },
+  name: "prefer-declare-function",
+  create(context) {
+    return {
+      // eslint-disable-next-line @typescript-eslint/naming-convention
+      "VariableDeclaration > VariableDeclarator"(node: TSESTree.VariableDeclarator) {
+        if (
+          node.id.typeAnnotation?.typeAnnotation.type === AST_NODE_TYPES.TSFunctionType &&
+          context.getFilename().endsWith(".d.ts")
+        ) {
+          context.report({
+            messageId: "variableFunction",
+            node: node.id,
+          });
+        }
+      },
+    };
+  },
+});
+
+export = rule;

packages/dtslint/test/prefer-declare-function.test.ts

@@ -0,0 +1,66 @@
+import { ESLintUtils } from "@typescript-eslint/utils";
+
+import * as preferDeclareFunction from "../src/rules/prefer-declare-function";
+
+const ruleTester = new ESLintUtils.RuleTester({
+  parser: "@typescript-eslint/parser",
+});
+
+ruleTester.run("prefer-declare-function", preferDeclareFunction, {
+  invalid: [
+    {
+      filename: "index.d.ts",
+      code: `export const example: () => void;`,
+      errors: [
+        {
+          column: 14,
+          endColumn: 33,
+          line: 1,
+          messageId: "variableFunction",
+        },
+      ],
+    },
+    {
+      filename: "index.d.ts",
+      code: `
+        namespace N {
+            export const example: () => void;
+        }
+`,
+      errors: [
+        {
+          column: 26,
+          endColumn: 45,
+          line: 3,
+          messageId: "variableFunction",
+        },
+      ],
+    },
+    {
+      filename: "index.d.ts",
+      code: `
+        namespace N {
+            const example: () => void;
+        }
+`,
+      errors: [
+        {
+          column: 19,
+          endColumn: 38,
+          line: 3,
+          messageId: "variableFunction",
+        },
+      ],
+    },
+  ],
+  valid: [
+    {
+      filename: "index.d.ts",
+      code: `function example(): void`,
+    },
+    {
+      filename: "test.ts",
+      code: `export const example: () => void;`,
+    },
+  ],
+});