Add no-nested-ternary rule (#296)

Co-authored-by: Sindre Sorhus <[email protected]>

Author: karaggeorge

docs/rules/no-nested-ternary.md

@@ -0,0 +1,49 @@
+# Disallow nested ternary expressions
+
+Improved version of the `[no-nested-ternary](https://eslint.org/docs/rules/no-nested-ternary)` ESLint rule, which allows cases where the nested ternary is only one level and wrapped in parens.
+
+
+## Fail
+
+```js
+const foo = i > 5 ? i < 100 ? true : false : true;
+const foo = i > 5 ? true : (i < 100 ? true : (i < 1000 ? true : false));
+```
+
+
+## Pass
+
+```js
+const foo = i > 5 ? (i < 100 ? true : false) : true;
+const foo = i > 5 ? (i < 100 ? true : false) : (i < 100 ? true : false);
+```
+
+
+## Partly fixable
+
+This rule is only fixable when the nesting is up to one level. The rule will wrap the nested ternary in parens:
+
+```js
+const foo = i > 5 ? i < 100 ? true : false : true
+```
+
+will get fixed to
+
+```js
+const foo = i > 5 ? (i < 100 ? true : false) : true
+```
+
+
+## Disabling ESLint `no-nested-ternary`
+
+We recomend disabling the ESLint `no-nested-ternary` rule in favor of this one:
+
+```json
+{
+	"rules": {
+		"no-nested-ternary": "off"
+	}
+}
+```
+
+The recommended preset that comes with this plugin already does this for you.

index.js

@@ -34,6 +34,8 @@ module.exports = {
 				'unicorn/no-for-loop': 'error',
 				'unicorn/no-hex-escape': 'error',
 				'unicorn/no-keyword-prefix': 'off',
+				'no-nested-ternary': 'off',
+				'unicorn/no-nested-ternary': 'error',
 				'unicorn/no-new-buffer': 'error',
 				'unicorn/no-process-exit': 'error',
 				'unicorn/no-unreadable-array-destructuring': 'error',

readme.md

@@ -52,6 +52,8 @@ Configure it in `package.json`.
 			"unicorn/no-for-loop": "error",
 			"unicorn/no-hex-escape": "error",
 			"unicorn/no-keyword-prefix": "off",
+			"no-nested-ternary": "off",
+			"unicorn/no-nested-ternary": "error",
 			"unicorn/no-new-buffer": "error",
 			"unicorn/no-process-exit": "error",
 			"unicorn/no-unreadable-array-destructuring": "error",
@@ -101,6 +103,7 @@ Configure it in `package.json`.
 - [no-for-loop](docs/rules/no-for-loop.md) - Do not use a `for` loop that can be replaced with a `for-of` loop. *(partly fixable)*
 - [no-hex-escape](docs/rules/no-hex-escape.md) - Enforce the use of Unicode escapes instead of hexadecimal escapes. *(fixable)*
 - [no-keyword-prefix](docs/rules/no-keyword-prefix.md) - Disallow identifiers starting with `new` or `class`.
+- [no-nested-ternary](docs/rules/no-nested-ternary.md) - Disallow nested ternary expressions. *(partly fixable)*
 - [no-new-buffer](docs/rules/no-new-buffer.md) - Enforce the use of `Buffer.from()` and `Buffer.alloc()` instead of the deprecated `new Buffer()`. *(fixable)*
 - [no-process-exit](docs/rules/no-process-exit.md) - Disallow `process.exit()`.
 - [no-unreadable-array-destructuring](docs/rules/no-unreadable-array-destructuring.md) - Disallow unreadable array destructuring.

rules/no-nested-ternary.js

@@ -0,0 +1,57 @@
+'use strict';
+const getDocsUrl = require('./utils/get-docs-url');
+
+const isParethesized = (sourceCode, node) => {
+	const previousToken = sourceCode.getTokenBefore(node);
+	const nextToken = sourceCode.getTokenAfter(node);
+
+	return Boolean(previousToken && nextToken) &&
+		previousToken.value === '(' &&
+		previousToken.end <= node.start &&
+		nextToken.value === ')' &&
+		nextToken.start >= node.end;
+};
+
+const create = context => {
+	const sourceCode = context.getSourceCode();
+
+	return {
+		ConditionalExpression: node => {
+			const nodesToCheck = [node.alternate, node.consequent];
+
+			for (const childNode of nodesToCheck) {
+				if (childNode.type !== 'ConditionalExpression') {
+					continue;
+				}
+
+				const message = 'Do not nest ternary expressions.';
+
+				// Nesting more than one level not allowed.
+				if (childNode.alternate.type === 'ConditionalExpression' || childNode.consequent.type === 'ConditionalExpression') {
+					context.report({node, message});
+					break;
+				} else if (!isParethesized(sourceCode, childNode)) {
+					context.report({
+						node: childNode,
+						message,
+						fix: fixer => [
+							fixer.insertTextBefore(childNode, '('),
+							fixer.insertTextAfter(childNode, ')')
+						]
+					});
+				}
+			}
+		}
+	};
+};
+
+module.exports = {
+	create,
+	meta: {
+		type: 'suggestion',
+		docs: {
+			url: getDocsUrl(__filename)
+		},
+		fixable: 'code'
+	}
+};

test/no-nested-ternary.js

@@ -0,0 +1,72 @@
+import test from 'ava';
+import avaRuleTester from 'eslint-ava-rule-tester';
+import rule from '../rules/no-nested-ternary';
+
+const ruleTester = avaRuleTester(test, {
+	env: {
+		es6: true
+	}
+});
+
+const errors = [
+	{
+		ruleId: 'no-nested-ternary',
+		message: 'Do not nest ternary expressions.'
+	}
+];
+
+ruleTester.run('new-error', rule, {
+	valid: [
+		'const foo = i > 5 ? true : false;',
+		'const foo = i > 5 ? true : (i < 100 ? true : false);',
+		'const foo = i > 5 ? (i < 100 ? true : false) : true;',
+		'const foo = i > 5 ? (i < 100 ? true : false) : (i < 100 ? true : false);',
+		'const foo = i > 5 ? true : (i < 100 ? FOO(i > 50 ? false : true) : false);',
+		'foo ? doBar() : doBaz();',
+		'var foo = bar === baz ? qux : quxx;'
+	],
+	invalid: [
+		{
+			code: 'const foo = i > 5 ? true : (i < 100 ? true : (i < 1000 ? true : false));',
+			errors
+		},
+		{
+			code: 'const foo = i > 5 ? true : (i < 100 ? (i > 50 ? false : true) : false);',
+			errors
+		},
+		{
+			code: 'const foo = i > 5 ? i < 100 ? true : false : true;',
+			output: 'const foo = i > 5 ? (i < 100 ? true : false) : true;',
+			errors
+		},
+		{
+			code: 'const foo = i > 5 ? i < 100 ? true : false : i < 100 ? true : false;',
+			output: 'const foo = i > 5 ? (i < 100 ? true : false) : (i < 100 ? true : false);',
+			errors: [
+				{
+					ruleId: 'no-nested-ternary',
+					column: 21
+				},
+				{
+					ruleId: 'no-nested-ternary',
+					column: 46
+				}
+			]
+		},
+		{
+			code: 'const foo = i > 5 ? true : i < 100 ? true : false;',
+			output: 'const foo = i > 5 ? true : (i < 100 ? true : false);',
+			errors
+		},
+		{
+			code: 'foo ? bar : baz === qux ? quxx : foobar;',
+			output: 'foo ? bar : (baz === qux ? quxx : foobar);',
+			errors
+		},
+		{
+			code: 'foo ? baz === qux ? quxx : foobar : bar;',
+			output: 'foo ? (baz === qux ? quxx : foobar) : bar;',
+			errors
+		}
+	]
+});