feat: create prefer-each rule

Author: G-Rath

README.md

@@ -226,6 +226,7 @@ installations requiring long-term consistency.
 | [no-test-return-statement](docs/rules/no-test-return-statement.md)           | Disallow explicitly returning from tests                            |                  |              |
 | [prefer-called-with](docs/rules/prefer-called-with.md)                       | Suggest using `toBeCalledWith()` or `toHaveBeenCalledWith()`        |                  |              |
 | [prefer-comparison-matcher](docs/rules/prefer-comparison-matcher.md)         | Suggest using the built-in comparison matchers                      |                  | ![fixable][] |
+| [prefer-each](docs/rules/prefer-each.md)                                     | Prefer using `.each` rather than manual loops                       |                  |              |
 | [prefer-equality-matcher](docs/rules/prefer-equality-matcher.md)             | Suggest using the built-in equality matchers                        |                  | ![suggest][] |
 | [prefer-expect-assertions](docs/rules/prefer-expect-assertions.md)           | Suggest using `expect.assertions()` OR `expect.hasAssertions()`     |                  | ![suggest][] |
 | [prefer-expect-resolves](docs/rules/prefer-expect-resolves.md)               | Prefer `await expect(...).resolves` over `expect(await ...)` syntax |                  | ![fixable][] |

docs/rules/prefer-each.md

@@ -0,0 +1,54 @@
+# Prefer using `.each` rather than manual loops (`prefer-each`)
+
+Reports where you might be able to use `.each` instead of native loops.
+
+## Rule details
+
+This rule triggers a warning if you use test case functions like `describe`,
+`test`, and `it`, in a native loop - generally you should be able to use `.each`
+instead which gives better output and makes it easier to run specific cases.
+
+Examples of **incorrect** code for this rule:
+
+```js
+for (const number of getNumbers()) {
+  it('is greater than five', function () {
+    expect(number).toBeGreaterThan(5);
+  });
+}
+
+for (const [input, expected] of data) {
+  beforeEach(() => setupSomething(input));
+
+  test(`results in ${expected}`, () => {
+    expect(doSomething()).toBe(expected);
+  });
+}
+```
+
+Examples of **correct** code for this rule:
+
+```js
+it.each(getNumbers())(
+  'only returns numbers that are greater than seven',
+  number => {
+    expect(number).toBeGreaterThan(7);
+  },
+);
+
+describe.each(data)('when input is %s', ([input, expected]) => {
+  beforeEach(() => setupSomething(input));
+
+  test(`results in ${expected}`, () => {
+    expect(doSomething()).toBe(expected);
+  });
+});
+
+// we don't warn on loops _in_ test functions because those typically involve
+// complex setup that is better done in the test function itself
+it('returns numbers that are greater than five', () => {
+  for (const number of getNumbers()) {
+    expect(number).toBeGreaterThan(5);
+  }
+});
+```

src/__tests__/__snapshots__/rules.test.ts.snap

@@ -1,15 +1,15 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
 exports[`rules should export configs that refer to actual rules 1`] = `
-Object {
-  "all": Object {
-    "env": Object {
+{
+  "all": {
+    "env": {
       "jest/globals": true,
     },
-    "plugins": Array [
+    "plugins": [
       "jest",
     ],
-    "rules": Object {
+    "rules": {
       "jest/consistent-test-it": "error",
       "jest/expect-expect": "error",
       "jest/max-expects": "error",
@@ -37,6 +37,7 @@ Object {
       "jest/no-test-return-statement": "error",
       "jest/prefer-called-with": "error",
       "jest/prefer-comparison-matcher": "error",
+      "jest/prefer-each": "error",
       "jest/prefer-equality-matcher": "error",
       "jest/prefer-expect-assertions": "error",
       "jest/prefer-expect-resolves": "error",
@@ -61,14 +62,14 @@ Object {
       "jest/valid-title": "error",
     },
   },
-  "recommended": Object {
-    "env": Object {
+  "recommended": {
+    "env": {
       "jest/globals": true,
     },
-    "plugins": Array [
+    "plugins": [
       "jest",
     ],
-    "rules": Object {
+    "rules": {
       "jest/expect-expect": "warn",
       "jest/no-commented-out-tests": "warn",
       "jest/no-conditional-expect": "error",
@@ -90,11 +91,11 @@ Object {
       "jest/valid-title": "error",
     },
   },
-  "style": Object {
-    "plugins": Array [
+  "style": {
+    "plugins": [
       "jest",
     ],
-    "rules": Object {
+    "rules": {
       "jest/no-alias-methods": "warn",
       "jest/prefer-to-be": "error",
       "jest/prefer-to-contain": "error",

src/__tests__/rules.test.ts

@@ -2,7 +2,7 @@ import { existsSync } from 'fs';
 import { resolve } from 'path';
 import plugin from '../';
 
-const numberOfRules = 50;
+const numberOfRules = 51;
 const ruleNames = Object.keys(plugin.rules);
 const deprecatedRules = Object.entries(plugin.rules)
   .filter(([, rule]) => rule.meta.deprecated)