typescript-eslint · bradzacher · Jan 10, 2020 · Jan 8, 2020 · Jan 10, 2020 · Jan 10, 2020
diff --git a/packages/eslint-plugin/README.md b/packages/eslint-plugin/README.md
@@ -105,6 +105,7 @@ Pro Tip: For larger codebases you may want to consider splitting our linting int
 | [`@typescript-eslint/class-name-casing`](./docs/rules/class-name-casing.md)                               | Require PascalCased class and interface names                                                                                                       | :heavy_check_mark: |          |                   |
 | [`@typescript-eslint/consistent-type-assertions`](./docs/rules/consistent-type-assertions.md)             | Enforces consistent usage of type assertions                                                                                                        | :heavy_check_mark: |          |                   |
 | [`@typescript-eslint/consistent-type-definitions`](./docs/rules/consistent-type-definitions.md)           | Consistent with type definition either `interface` or `type`                                                                                        |                    | :wrench: |                   |
+| [`@typescript-eslint/default-param-last`](./docs/rules/default-param-last.md)           | Enforce default parameters to be last                                                                                        |                    | |                   |
 | [`@typescript-eslint/explicit-function-return-type`](./docs/rules/explicit-function-return-type.md)       | Require explicit return types on functions and class methods                                                                                        | :heavy_check_mark: |          |                   |
 | [`@typescript-eslint/explicit-member-accessibility`](./docs/rules/explicit-member-accessibility.md)       | Require explicit accessibility modifiers on class properties and methods                                                                            |                    |          |                   |
 | [`@typescript-eslint/func-call-spacing`](./docs/rules/func-call-spacing.md)                               | Require or disallow spacing between function identifiers and their invocations                                                                      |                    | :wrench: |                   |

diff --git a/packages/eslint-plugin/docs/rules/default-param-last.md b/packages/eslint-plugin/docs/rules/default-param-last.md
@@ -0,0 +1,29 @@
+# Enforce default parameters to be last (`default-param-last`)
+
+## Rule Details
+
+This rule enforces default or optional parameters to be the last of parameters.
+
+Examples of **incorrect** code for this rule:
+
+```ts
+/* eslint @typescript-eslint/default-param-last: ["error"] */
+
+function f(a = 0, b: number) {}
+function f(a: number, b = 0, c?: number) {}
+function f(a: number, b = 0, c: number) {}
+function f(a: number, b?: number, c: number) {}
+```
+
+Examples of **correct** code for this rule:
+
+```ts
+/* eslint @typescript-eslint/default-param-last: ["error"] */
+
+function f(a = 0) {}
+function f(a: number, b = 0) {}
+function f(a: number, b?: number) {}
+function f(a: number, b?: number, c = 0) {}
+```
+
+<sup>Taken with ❤️ [from ESLint core](https://github.com/eslint/eslint/blob/master/docs/rules/default-param-last.md)</sup>
diff --git a/packages/eslint-plugin/src/configs/all.json b/packages/eslint-plugin/src/configs/all.json
@@ -13,6 +13,7 @@
     "@typescript-eslint/class-name-casing": "error",
     "@typescript-eslint/consistent-type-assertions": "error",
     "@typescript-eslint/consistent-type-definitions": "error",
+    "@typescript-eslint/default-param-last": "error",
     "@typescript-eslint/explicit-function-return-type": "error",
     "@typescript-eslint/explicit-member-accessibility": "error",
     "func-call-spacing": "off",

diff --git a/packages/eslint-plugin/src/rules/default-param-last.ts b/packages/eslint-plugin/src/rules/default-param-last.ts
@@ -0,0 +1,76 @@
+import { createRule } from '../util';
+import {
+  TSESTree,
+  AST_NODE_TYPES,
+} from '@typescript-eslint/experimental-utils';
+
+export default createRule({
+  name: 'default-param-last',
+  meta: {
+    type: 'suggestion',
+    docs: {
+      description: 'Enforce default parameters to be last',
+      category: 'Best Practices',
+      recommended: false,
+    },
+    schema: [],
+    messages: {
+      shouldBeLast: 'Default parameters should be last.',
+    },
+  },
+  defaultOptions: [],
+  create(context) {
+    /**
+     * checks if node is optional parameter
+     * @param node the node to be evaluated
+     * @private
+     */
+    function isOptionalParam(node: TSESTree.Parameter): boolean {
+      return 'optional' in node && node.optional === true;
+    }
+
+    /**
+     * checks if node is plain parameter
+     * @param node the node to be evaluated
+     * @private
+     */
+    function isPlainParam(node: TSESTree.Parameter): boolean {
+      return !(
+        node.type === AST_NODE_TYPES.AssignmentPattern ||
+        node.type === AST_NODE_TYPES.RestElement ||
+        isOptionalParam(node)
+      );
+    }
+
+    function checkDefaultParamLast(
+      node:
+        | TSESTree.ArrowFunctionExpression
+        | TSESTree.FunctionDeclaration
+        | TSESTree.FunctionExpression,
+    ): void {
+      let hasSeenPlainParam = false;
+      for (let i = node.params.length - 1; i >= 0; i--) {
+        const param = node.params[i];
+
+        if (isPlainParam(param)) {
+          hasSeenPlainParam = true;
+          continue;
+        }
+
+        if (
+          hasSeenPlainParam &&
+          (isOptionalParam(param) ||
+            param.type === AST_NODE_TYPES.AssignmentPattern)
+        ) {
+          context.report({ node: param, messageId: 'shouldBeLast' });
+        }
+      }
+    }
+
+    return {
+      ArrowFunctionExpression: checkDefaultParamLast,
+      FunctionDeclaration: checkDefaultParamLast,
+      FunctionExpression: checkDefaultParamLast,
+    };
+  },
+});
diff --git a/packages/eslint-plugin/src/rules/index.ts b/packages/eslint-plugin/src/rules/index.ts
@@ -8,6 +8,7 @@ import camelcase from './camelcase';
 import classNameCasing from './class-name-casing';
 import consistentTypeAssertions from './consistent-type-assertions';
 import consistentTypeDefinitions from './consistent-type-definitions';
+import defaultParamLast from './default-param-last';
 import explicitFunctionReturnType from './explicit-function-return-type';
 import explicitMemberAccessibility from './explicit-member-accessibility';
 import funcCallSpacing from './func-call-spacing';
@@ -87,6 +88,7 @@ export default {
   'class-name-casing': classNameCasing,
   'consistent-type-assertions': consistentTypeAssertions,
   'consistent-type-definitions': consistentTypeDefinitions,
+  'default-param-last': defaultParamLast,
   'explicit-function-return-type': explicitFunctionReturnType,
   'explicit-member-accessibility': explicitMemberAccessibility,
   'func-call-spacing': funcCallSpacing,