Add number-literal-case rule (fixes #55) (#57)

* add number-literal-case rule - fixes #55

* process feedback

* remove meta

* fix description

docs/rules/number-literal-case.md

@@ -0,0 +1,35 @@
+# Enforce lowercase identifier and uppercase value for number literals
+
+Enforces a convention of defining number literals where the literal identifier is written in lowercase and the value in uppercase. Differentiating the casing of the identifier and value clearly separates them and makes your code more readable.
+
+
+## Fail
+
+```js
+const foo = 0XFF;
+const foo = 0xff;
+const foo = 0Xff;
+```
+
+```js
+const foo = 0B11;
+```
+
+```js
+const foo = 0O10;
+```
+
+
+## Pass
+
+```js
+const foo = 0xFF;
+```
+
+```js
+const foo = 0b11;
+```
+
+```js
+const foo = 0o10;
+```

index.js

@@ -18,7 +18,8 @@ module.exports = {
 				'unicorn/filename-case': ['error', {case: 'kebabCase'}],
 				'unicorn/no-abusive-eslint-disable': 'error',
 				'unicorn/no-process-exit': 'error',
-				'unicorn/throw-new-error': 'error'
+				'unicorn/throw-new-error': 'error',
+				'unicorn/number-literal-case': 'error'
 			}
 		}
 	}

readme.md

@@ -38,7 +38,8 @@ Configure it in `package.json`.
 			"unicorn/filename-case": ["error", {"case": "kebabCase"}],
 			"unicorn/no-abusive-eslint-disable": "error",
 			"unicorn/no-process-exit": "error",
-			"unicorn/throw-new-error": "error"
+			"unicorn/throw-new-error": "error",
+			"unicorn/number-literal-case": "error"
 		}
 	}
 }
@@ -53,6 +54,7 @@ Configure it in `package.json`.
 - [no-abusive-eslint-disable](docs/rules/no-abusive-eslint-disable.md) - Enforce specifying rules to disable in `eslint-disable` comments.
 - [no-process-exit](docs/rules/no-process-exit.md) - Disallow `process.exit()`.
 - [throw-new-error](docs/rules/throw-new-error.md) - Require `new` when throwing an error. *(fixable)*
+- [number-literal-case](docs/rules/number-literal-case.md) - Enforce lowercase identifier and uppercase value for number literals. *(fixable)*
 
 
 ## Recommended config

rules/number-literal-case.js

@@ -0,0 +1,35 @@
+'use strict';
+const fix = value => {
+	if (!/^0[a-zA-Z]/.test(value)) {
+		return value;
+	}
+
+	const indicator = value[1].toLowerCase();
+	const val = value.slice(2).toUpperCase();
+
+	return `0${indicator}${val}`;
+};
+
+const create = context => {
+	return {
+		Literal: node => {
+			const value = node.raw;
+			const fixedValue = fix(value);
+
+			if (value !== fixedValue) {
+				context.report({
+					node,
+					message: 'Invalid number literal casing',
+					fix: fixer => fixer.replaceText(node, fixedValue)
+				});
+			}
+		}
+	};
+};
+
+module.exports = {
+	meta: {
+		fixable: 'code'
+	},
+	create
+};

test/number-literal-case.js

@@ -0,0 +1,75 @@
+import test from 'ava';
+import avaRuleTester from 'eslint-ava-rule-tester';
+import rule from '../rules/number-literal-case';
+
+const ruleTester = avaRuleTester(test, {
+	env: {
+		es6: true
+	},
+	parserOptions: {
+		sourceType: 'module'
+	}
+});
+
+const error = {
+	ruleId: 'number-literal-case',
+	message: 'Invalid number literal casing'
+};
+
+ruleTester.run('number-literal-case', rule, {
+	valid: [
+		'const foo = 0xFF',
+		'const foo = 0b11',
+		'const foo = 0o10',
+		`const foo = '0Xff'`
+	],
+	invalid: [
+		{
+			code: 'const foo = 0XFF',
+			errors: [error],
+			output: 'const foo = 0xFF'
+		},
+		{
+			code: 'const foo = 0xff',
+			errors: [error],
+			output: 'const foo = 0xFF'
+		},
+		{
+			code: 'const foo = 0Xff',
+			errors: [error],
+			output: 'const foo = 0xFF'
+		},
+		{
+			code: 'const foo = 0Xff',
+			errors: [error],
+			output: 'const foo = 0xFF'
+		},
+		{
+			code: 'const foo = 0B11',
+			errors: [error],
+			output: 'const foo = 0b11'
+		},
+		{
+			code: 'const foo = 0O10',
+			errors: [error],
+			output: 'const foo = 0o10'
+		},
+		{
+			code: `
+				const foo = 255;
+
+				if (foo === 0xff) {
+					console.log('invalid');
+				}
+			`,
+			errors: [error],
+			output: `
+				const foo = 255;
+
+				if (foo === 0xFF) {
+					console.log('invalid');
+				}
+			`
+		}
+	]
+});