Add text-encoding-identifier-case rule

configs/recommended.js

@@ -105,6 +105,7 @@ module.exports = {
 		'unicorn/require-post-message-target-origin': 'off',
 		'unicorn/string-content': 'off',
 		'unicorn/template-indent': 'warn',
+		'unicorn/text-encoding-identifier-case': 'error',
 		'unicorn/throw-new-error': 'error',
 		...require('./conflicting-rules.js').rules,
 	},

docs/rules/text-encoding-identifier-case.md

@@ -0,0 +1,39 @@
+# Enforce consistent case for text encoding identifiers
+
+<!-- Do not manually modify RULE_NOTICE part. Run: `npm run generate-rule-notices` -->
+<!-- RULE_NOTICE -->
+✅ *This rule is part of the [recommended](https://github.com/sindresorhus/eslint-plugin-unicorn#recommended-config) config.*
+
+💡 *This rule provides [suggestions](https://eslint.org/docs/developer-guide/working-with-rules#providing-suggestions).*
+<!-- /RULE_NOTICE -->
+
+- Enforce `'utf8'` for [UTF-8](https://en.wikipedia.org/wiki/UTF-8) encoding.
+- Enforce `'ascii'` for [ASCII](https://en.wikipedia.org/wiki/ASCII) encoding.
+
+## Fail
+
+```js
+await fs.readFile(file, 'UTF-8');
+```
+
+```js
+await fs.readFile(file, 'ASCII');
+```
+
+```js
+const string = buffer.toString('utf-8');
+```
+
+## Pass
+
+```js
+await fs.readFile(file, 'utf8');
+```
+
+```js
+await fs.readFile(file, 'ascii');
+```
+
+```js
+const string = buffer.toString('utf8');
+```

readme.md

@@ -150,6 +150,7 @@ Each rule has emojis denoting:
 | [require-post-message-target-origin](docs/rules/require-post-message-target-origin.md) | Enforce using the `targetOrigin` argument with `window.postMessage()`. |  |  | 💡 |
 | [string-content](docs/rules/string-content.md) | Enforce better string content. |  | 🔧 | 💡 |
 | [template-indent](docs/rules/template-indent.md) | Fix whitespace-insensitive template indentation. | ✅ | 🔧 |  |
+| [text-encoding-identifier-case](docs/rules/text-encoding-identifier-case.md) | Enforce consistent case for text encoding identifiers. | ✅ |  | 💡 |
 | [throw-new-error](docs/rules/throw-new-error.md) | Require `new` when throwing an error. | ✅ | 🔧 |  |
 <!-- /RULES_TABLE -->
 

rules/text-encoding-identifier-case.js

@@ -0,0 +1,69 @@
+'use strict';
+const {replaceStringLiteral} = require('./fix/index.js');
+
+const MESSAGE_ID_ERROR = 'text-encoding-identifier/error';
+const MESSAGE_ID_SUGGESTION = 'text-encoding-identifier/suggestion';
+const messages = {
+	[MESSAGE_ID_ERROR]: 'Prefer `{{replacement}}` over `{{value}}`.',
+	[MESSAGE_ID_SUGGESTION]: 'Replace `{{value}}` with `{{replacement}}`.',
+};
+
+const getReplacement = encoding => {
+	switch (encoding.toLowerCase()) {
+		case 'utf8':
+		case 'utf-8':
+			return 'utf8';
+		case 'ascii':
+			return 'ascii';
+		// No default
+	}
+};
+
+/** @param {import('eslint').Rule.RuleContext} context */
+const create = () => ({
+	Literal(node) {
+		if (typeof node.value !== 'string') {
+			return;
+		}
+
+		const {raw} = node;
+		const value = raw.slice(1, -1);
+
+		const replacement = getReplacement(value);
+		if (!replacement || replacement === value) {
+			return;
+		}
+
+		const messageData = {
+			value,
+			replacement,
+		};
+
+		return {
+			node,
+			messageId: MESSAGE_ID_ERROR,
+			data: messageData,
+			suggest: [
+				{
+					messageId: MESSAGE_ID_SUGGESTION,
+					data: messageData,
+					/** @param {import('eslint').Rule.RuleFixer} fixer */
+					fix: fixer => replaceStringLiteral(fixer, node, replacement),
+				},
+			],
+		};
+	},
+});
+
+/** @type {import('eslint').Rule.RuleModule} */
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			description: 'Enforce consistent case for text encoding identifiers.',
+		},
+		hasSuggestions: true,
+		messages,
+	},
+};

test/run-rules-on-codebase/lint.mjs

@@ -35,6 +35,8 @@ const eslint = new ESLint({
 			'unicorn/prefer-string-replace-all': 'off',
 			'unicorn/prefer-top-level-await': 'off',
 			'unicorn/prefer-at': 'off',
+			// TODO: Turn this on when `xo` updated `eslint-plugin-unicorn`
+			'unicorn/text-encoding-identifier-case': 'off',
 		},
 		overrides: [
 			{

test/snapshots/text-encoding-identifier-case.mjs.md

@@ -0,0 +1,89 @@
+# Snapshot report for `test/text-encoding-identifier-case.mjs`
+
+The actual snapshot is saved in `text-encoding-identifier-case.mjs.snap`.
+
+Generated by [AVA](https://avajs.dev).
+
+## Invalid #1
+      1 | "UTF-8"
+
+> Error 1/1
+
+    `␊
+    > 1 | "UTF-8"␊
+        | ^^^^^^^ Prefer \`utf8\` over \`UTF-8\`.␊
+    ␊
+    --------------------------------------------------------------------------------␊
+    Suggestion 1/1: Replace \`UTF-8\` with \`utf8\`.␊
+      1 | "utf8"␊
+    `
+
+## Invalid #2
+      1 | "utf-8"
+
+> Error 1/1
+
+    `␊
+    > 1 | "utf-8"␊
+        | ^^^^^^^ Prefer \`utf8\` over \`utf-8\`.␊
+    ␊
+    --------------------------------------------------------------------------------␊
+    Suggestion 1/1: Replace \`utf-8\` with \`utf8\`.␊
+      1 | "utf8"␊
+    `
+
+## Invalid #3
+      1 | 'utf-8'
+
+> Error 1/1
+
+    `␊
+    > 1 | 'utf-8'␊
+        | ^^^^^^^ Prefer \`utf8\` over \`utf-8\`.␊
+    ␊
+    --------------------------------------------------------------------------------␊
+    Suggestion 1/1: Replace \`utf-8\` with \`utf8\`.␊
+      1 | 'utf8'␊
+    `
+
+## Invalid #4
+      1 | "Utf8"
+
+> Error 1/1
+
+    `␊
+    > 1 | "Utf8"␊
+        | ^^^^^^ Prefer \`utf8\` over \`Utf8\`.␊
+    ␊
+    --------------------------------------------------------------------------------␊
+    Suggestion 1/1: Replace \`Utf8\` with \`utf8\`.␊
+      1 | "utf8"␊
+    `
+
+## Invalid #5
+      1 | "ASCII"
+
+> Error 1/1
+
+    `␊
+    > 1 | "ASCII"␊
+        | ^^^^^^^ Prefer \`ascii\` over \`ASCII\`.␊
+    ␊
+    --------------------------------------------------------------------------------␊
+    Suggestion 1/1: Replace \`ASCII\` with \`ascii\`.␊
+      1 | "ascii"␊
+    `
+
+## Invalid #6
+      1 | await fs.readFile(file, "UTF-8",)
+
+> Error 1/1
+
+    `␊
+    > 1 | await fs.readFile(file, "UTF-8",)␊
+        |                         ^^^^^^^ Prefer \`utf8\` over \`UTF-8\`.␊
+    ␊
+    --------------------------------------------------------------------------------␊
+    Suggestion 1/1: Replace \`UTF-8\` with \`utf8\`.␊
+      1 | await fs.readFile(file, "utf8",)␊
+    `

test/text-encoding-identifier-case.mjs

@@ -0,0 +1,24 @@
+import {getTester} from './utils/test.mjs';
+
+const {test} = getTester(import.meta);
+
+test.snapshot({
+	valid: [
+		'`UTF-8`',
+		'"utf8"',
+		'"utf+8"',
+		'"   utf8   "',
+		'\'utf8\'',
+		'"\\u0055tf8"',
+		'const ASCII = 1',
+		'const UTF8 = 1',
+	],
+	invalid: [
+		'"UTF-8"',
+		'"utf-8"',
+		'\'utf-8\'',
+		'"Utf8"',
+		'"ASCII"',
+		'await fs.readFile(file, "UTF-8",)',
+	],
+});