Add no-array-instanceof rule (fixes #11) (#58)

Author: pvskarthikeya

docs/rules/no-array-instanceof.md

@@ -0,0 +1,17 @@
+# Require  `Array.isArray()` check instead of `instanceof Array`
+
+While the `instanceof Array` works, it is unreliable across environments Eg: multi-frame DOM environments (iframes). Hence, `Array.isArray()` is recommended.
+
+## Fail
+
+```js
+array instanceof Array;
+[1,2,3] instanceof Array;
+```
+
+## Pass
+
+```js
+Array.isArray(array);
+Array.isArray([1,2,3]);
+```

index.js

@@ -19,6 +19,7 @@ module.exports = {
 				'unicorn/no-abusive-eslint-disable': 'error',
 				'unicorn/no-process-exit': 'error',
 				'unicorn/throw-new-error': 'error',
+				'unicorn/no-array-instanceof': 'error',
 				'unicorn/number-literal-case': 'error'
 			}
 		}

readme.md

@@ -39,7 +39,8 @@ Configure it in `package.json`.
 			"unicorn/no-abusive-eslint-disable": "error",
 			"unicorn/no-process-exit": "error",
 			"unicorn/throw-new-error": "error",
-			"unicorn/number-literal-case": "error"
+			"unicorn/number-literal-case": "error",
+			"unicorn/no-array-instanceof": "error"
 		}
 	}
 }
@@ -55,6 +56,7 @@ Configure it in `package.json`.
 - [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)*
+- [no-array-instanceof](docs/rules/no-array-instanceof.md) - Disallow `instanceof Array`, instead use `Array.isArray()`. *(fixable)*
 
 
 ## Recommended config

rules/no-array-instanceof.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const create = context => ({
+	BinaryExpression: node => {
+		if (node.operator === 'instanceof' && node.right.type === 'Identifier' && node.right.name === 'Array') {
+			// Get the source code and extract the left part.
+			const arraySourceCode = context.getSourceCode().text.slice(node.left.range[0], node.left.range[1]);
+			context.report({
+				node,
+				message: 'Use `Array.isArray()` instead of `instanceof Array`',
+				fix: fixer => fixer.replaceText(node, `Array.isArray(${arraySourceCode})`)
+			});
+		}
+	}
+});
+
+module.exports = {
+	create,
+	meta: {
+		fixable: 'code'
+	}
+};

test/no-array-instanceof.js

@@ -0,0 +1,59 @@
+import test from 'ava';
+import avaRuleTester from 'eslint-ava-rule-tester';
+import rule from '../rules/no-array-instanceof';
+
+const ruleTester = avaRuleTester(test, {
+	env: {
+		es6: true
+	}
+});
+
+const errors = [{
+	ruleId: 'no-array-instanceof',
+	message: 'Use `Array.isArray()` instead of `instanceof Array`'
+}];
+
+ruleTester.run('avoid-array-instanceof', rule, {
+	valid: [
+		'Array.isArray(arr)',
+		'arr instanceof Object',
+		'arr instanceof array',
+		'a instanceof \'array\'',
+		'a instanceof ArrayA',
+		'a.x[2] instanceof foo()',
+		'Array.isArray([1,2,3]) === true',
+		'"arr instanceof Array"'
+	],
+	invalid: [
+		{
+			code: 'arr instanceof Array',
+			output: 'Array.isArray(arr)',
+			errors
+		},
+		{
+			code: '[] instanceof Array',
+			output: 'Array.isArray([])',
+			errors
+		},
+		{
+			code: '[1,2,3] instanceof Array === true',
+			output: 'Array.isArray([1,2,3]) === true',
+			errors
+		},
+		{
+			code: 'fun.call(1, 2, 3) instanceof Array',
+			output: 'Array.isArray(fun.call(1, 2, 3))',
+			errors
+		},
+		{
+			code: 'obj.arr instanceof Array',
+			output: 'Array.isArray(obj.arr)',
+			errors
+		},
+		{
+			code: 'foo.bar[2] instanceof Array',
+			output: 'Array.isArray(foo.bar[2])',
+			errors
+		}
+	]
+});