Add lowercase-description rule

docs/rules/lowercase-description.md

@@ -0,0 +1,21 @@
+# Enforce lowercase descriptions in it() (lowercase-description)
+
+## Rule details
+
+Enforce `it()` tests to have descriptions that begin with a lowercase letter. This provides more readable test failures. This rule is not enabled by default.
+
+The following pattern is considered a warning:
+
+```js
+it('Adds 1 + 2 to equal 3', () => {
+  expect(sum(1, 2)).toBe(3);
+});
+```
+
+The following pattern is not considered a warning:
+
+```js
+it('adds 1 + 2 to equal 3', () => {
+  expect(sum(1, 2)).toBe(3);
+});
+```

index.js

@@ -1,5 +1,6 @@
 'use strict';
 
+const lowercaseDescription = require('./rules/lowercase_description');
 const noDisabledTests = require('./rules/no_disabled_tests');
 const noFocusedTests = require('./rules/no_focused_tests');
 const noIdenticalTitle = require('./rules/no_identical_title');
@@ -55,6 +56,7 @@ module.exports = {
     '.snap': snapshotProcessor,
   },
   rules: {
+    'lowercase-description': lowercaseDescription,
     'no-disabled-tests': noDisabledTests,
     'no-focused-tests': noFocusedTests,
     'no-identical-title': noIdenticalTitle,

rules/__tests__/lowercase_description.test.js

@@ -0,0 +1,40 @@
+'use strict';
+
+const RuleTester = require('eslint').RuleTester;
+const rules = require('../..').rules;
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 6,
+  },
+});
+
+const errMessage = 'it() description should begin with lowercase';
+const errors = [{ message: errMessage, column: 1, line: 1 }];
+
+ruleTester.run('lowercase-description', rules['lowercase-description'], {
+  valid: [
+    "it('foo', function () {})",
+    'it("foo", function () {})',
+    'it(`foo`, function () {})',
+    'it("<Foo/>", function () {})',
+    'it("123 foo", function () {})',
+    'describe("foo", function () {})',
+    'test("foo", function () {})',
+  ],
+
+  invalid: [
+    {
+      code: "it('Foo', function () {})",
+      errors,
+    },
+    {
+      code: 'it("Foo", function () {})',
+      errors,
+    },
+    {
+      code: 'it(`Foo`, function () {})',
+      errors,
+    },
+  ],
+});

rules/lowercase_description.js

@@ -0,0 +1,49 @@
+'use strict';
+
+const ruleMsg = 'it() description should begin with lowercase';
+
+const isItFunction = node => {
+  return (
+    node.type === 'CallExpression' && node.callee && node.callee.name === 'it'
+  );
+};
+
+const isItDescription = node => {
+  const foo =
+    node.arguments &&
+    node.arguments[0] &&
+    (node.arguments[0].type === 'Literal' ||
+      node.arguments[0].type === 'TemplateLiteral');
+  return foo;
+};
+
+const testDescription = node => {
+  const { type } = node.arguments[0];
+  if (type === 'Literal') {
+    return node.arguments[0].value;
+  }
+  if (type === 'TemplateLiteral') {
+    return node.arguments[0].quasis[0].value.raw;
+  }
+};
+
+const descriptionBeginsWithLowerCase = node => {
+  if (isItFunction(node) && isItDescription(node)) {
+    const description = testDescription(node);
+    return description[0] === description[0].toLowerCase();
+  }
+  return true;
+};
+
+module.exports = context => {
+  return {
+    CallExpression(node) {
+      if (!descriptionBeginsWithLowerCase(node)) {
+        context.report({
+          message: ruleMsg,
+          node: node,
+        });
+      }
+    },
+  };
+};