Add explicit-length-check rule (fixes #12) (#45)

Author: jfmengels

docs/rules/explicit-length-check.md

@@ -0,0 +1,21 @@
+# Enforce explicitly comparing the `length` property of a value
+
+Enforce explicitly checking the length of a value array in an `if` condition, rather than checking the truthiness of the length.
+
+## Fail
+
+```js
+if (string.length) {}
+if (array.length) {}
+if (!array.length) {}
+```
+
+
+## Pass
+
+```js
+if (string.length > 0) {}
+if (array.length > 0) {}
+if (array.length !== 0) {}
+if (array.length === 0) {}
+```

index.js

@@ -14,6 +14,7 @@ module.exports = {
 			},
 			rules: {
 				'xo/catch-error-name': ['error', {name: 'err'}],
+				'xo/explicit-length-check': 'error',
 				'xo/filename-case': ['error', {case: 'kebabCase'}],
 				'xo/no-abusive-eslint-disable': 'error',
 				'xo/no-process-exit': 'error',

readme.md

@@ -32,6 +32,7 @@ Configure it in `package.json`.
 		],
 		"rules": {
 			"xo/catch-error-name": ["error", {"name": "err"}],
+			"xo/explicit-length-check": "error",
 			"xo/filename-case": ["error", {"case": "kebabCase"}],
 			"xo/no-abusive-eslint-disable": "error",
 			"xo/no-process-exit": "error",
@@ -45,6 +46,7 @@ Configure it in `package.json`.
 ## Rules
 
 - [catch-error-name](docs/rules/catch-error-name.md) - Enforce a specific parameter name in catch clauses.
+- [explicit-length-check](docs/rules/explicit-length-check.md) - Enforce explicitly comparing the `length` property of a value.
 - [filename-case](docs/rules/filename-case.md) - Enforce a case style for filenames.
 - [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()`.

rules/explicit-length-check.js

@@ -0,0 +1,32 @@
+'use strict';
+
+function checkExpression(context, node) {
+	if (node.type === 'LogicalExpression') {
+		checkExpression(context, node.left);
+		checkExpression(context, node.right);
+		return;
+	}
+
+	if (node.type === 'UnaryExpression' && node.operator === '!') {
+		checkExpression(context, node.argument);
+		return;
+	}
+
+	if (node.type === 'MemberExpression' &&
+		node.property.type === 'Identifier' &&
+		node.property.name === 'length'
+	) {
+		context.report({
+			node,
+			message: '`length` property should be compared to a value'
+		});
+	}
+}
+
+module.exports = context => {
+	return {
+		IfStatement: node => {
+			checkExpression(context, node.test);
+		}
+	};
+};

test/explicit-length-check.js

@@ -0,0 +1,65 @@
+import test from 'ava';
+import avaRuleTester from 'eslint-ava-rule-tester';
+import rule from '../rules/explicit-length-check';
+
+const ruleTester = avaRuleTester(test, {
+	env: {
+		es6: true
+	}
+});
+
+const errors = [{
+	ruleId: 'explicit-length-check',
+	message: '`length` property should be compared to a value'
+}];
+
+ruleTester.run('explicit-length-check', rule, {
+	valid: [
+		'array.foo',
+		'array.length',
+		'array.length === 0',
+		'array.length !== 0',
+		'array.length > 0',
+		'if (array.foo) {}',
+		'if (length) {}',
+		'if ([].length > 0) {}',
+		'if ("".length > 0) {}',
+		'if (array.length === 0) {}',
+		'if (array.length !== 0) {}',
+		'if (array.length !== 0 && array[0] === 1) {}'
+	],
+	invalid: [
+		{
+			code: 'if ([].length) {}',
+			errors
+		},
+		{
+			code: 'if ("".length) {}',
+			errors
+		},
+		{
+			code: 'if (array.length) {}',
+			errors
+		},
+		{
+			code: 'if (!array.length) {}',
+			errors
+		},
+		{
+			code: 'if (array.foo.length) {}',
+			errors
+		},
+		{
+			code: 'if (!!array.length) {}',
+			errors
+		},
+		{
+			code: 'if (array.length && array[0] === 1) {}',
+			errors
+		},
+		{
+			code: 'if (array[0] === 1 || array.length) {}',
+			errors
+		}
+	]
+});