Add prefer-to-have-length rule

CHANGELOG.md

@@ -19,6 +19,7 @@
 * `[jest-config]` Include error message for `preset` json ([#4766](https://github.com/facebook/jest/pull/4766))
 
 ### Features
+* `[eslint-plugin-jest]` Add `prefer-to-have-length` lint rule. ([#4771](https://github.com/facebook/jest/pull/4771))
 * `[jest-environment-jsdom]` [**BREAKING**] Upgrade to JSDOM@11 ([#4770](https://github.com/facebook/jest/pull/4770))
 * `[jest-environment-*]` [**BREAKING**] Add Async Test Environment APIs, dispose is now teardown ([#4506](https://github.com/facebook/jest/pull/4506))
 * `[jest-cli]` Add an option to clear the cache ([#4430](https://github.com/facebook/jest/pull/4430))

packages/eslint-plugin-jest/docs/rules/prefer-to-have-length.md

@@ -0,0 +1,27 @@
+# Suggest using  `toHaveLength()` (prefer-to-have-length)
+
+In order to have a better failure message, `toHaveLength()` should be used upon asserting expections on object's length property.
+
+## Rule details
+
+This rule triggers a warning if `toBe()` is used to assert object's length property.
+
+```js
+expect(files.length).toBe(1);
+```
+
+This rule is enabled by default.
+
+### Default configuration
+
+The following pattern is considered warning:
+
+```js
+expect(files.length).toBe(1);
+```
+
+The following pattern is not warning:
+
+```js
+expect(files).toHaveLength(1);
+```

packages/eslint-plugin-jest/src/index.js

@@ -10,6 +10,7 @@
 import noDisabledTests from './rules/no_disabled_tests';
 import noFocusedTests from './rules/no_focused_tests';
 import noIdenticalTitle from './rules/no_identical_title';
+import preferToHaveLength from './rules/prefer_to_have_length';
 import validExpect from './rules/valid_expect';
 
 module.exports = {
@@ -19,6 +20,7 @@ module.exports = {
         'jest/no-disabled-tests': 'warn',
         'jest/no-focused-tests': 'error',
         'jest/no-identical-title': 'error',
+        'jest/prefer-to-have-length': 'warn',
         'jest/valid-expect': 'error',
       },
     },
@@ -50,6 +52,7 @@ module.exports = {
     'no-disabled-tests': noDisabledTests,
     'no-focused-tests': noFocusedTests,
     'no-identical-title': noIdenticalTitle,
+    'prefer-to-have-length': preferToHaveLength,
     'valid-expect': validExpect,
   },
 };

packages/eslint-plugin-jest/src/rules/__tests__/prefer_to_have_length.test.js

@@ -0,0 +1,34 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+/* eslint-disable sort-keys */
+
+'use strict';
+
+import {RuleTester} from 'eslint';
+const {rules} = require('../../');
+
+const ruleTester = new RuleTester();
+
+ruleTester.run('prefer_to_have_length', rules['prefer-to-have-length'], {
+  valid: ['expect(files).toHaveLength(1);', "expect(files.name).toBe('file');"],
+
+  invalid: [
+    {
+      code: 'expect(files.length).toBe(1);',
+      errors: [
+        {
+          message: 'Use toHaveLength() instead',
+          column: 22,
+          line: 1,
+        },
+      ],
+    },
+  ],
+});

packages/eslint-plugin-jest/src/rules/prefer_to_have_length.js

@@ -0,0 +1,39 @@
+/**
+ * Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow
+ */
+
+import type {EslintContext, CallExpression} from './types';
+
+export default (context: EslintContext) => {
+  return {
+    CallExpression(node: CallExpression) {
+      const calleeName = node.callee.name;
+
+      if (
+        calleeName === 'expect' &&
+        node.arguments.length == 1 &&
+        node.parent &&
+        node.parent.type === 'MemberExpression' &&
+        node.parent.parent
+      ) {
+        const parentProperty = node.parent.property;
+        const propertyName = parentProperty.name;
+
+        if (
+          propertyName === 'toBe' &&
+          node.arguments[0].property.name === 'length'
+        ) {
+          context.report({
+            message: 'Use toHaveLength() instead',
+            node: parentProperty,
+          });
+        }
+      }
+    },
+  };
+};

packages/eslint-plugin-jest/src/rules/types.js

@@ -44,6 +44,7 @@ export type Literal = {
   value?: string,
   rawValue?: string,
   parent: ParentNode,
+  property: Identifier,
   loc: NodeLocation,
 };