Add new rule no-leaking-state-in-program-scope

README.md

@@ -119,6 +119,7 @@ The `--fix` option on the command line automatically fixes problems reported by
 | :wrench: | [no-ember-super-in-es-classes](./docs/rules/no-ember-super-in-es-classes.md) | Prevents use of `this._super` in ES class methods |
 | :white_check_mark: | [no-ember-testing-in-module-scope](./docs/rules/no-ember-testing-in-module-scope.md) | Prevents use of Ember.testing in module scope |
 |  | [no-invalid-debug-function-arguments](./docs/rules/no-invalid-debug-function-arguments.md) | Catch usages of Ember's `assert()` / `warn()` / `deprecate()` functions that have the arguments passed in the wrong order. |
+|  | [no-leaking-state-in-program-scope](./docs/rules/no-leaking-state-in-program-scope.md) | Do not allow anything else than constants in global module namespace. |
 | :white_check_mark: | [no-side-effects](./docs/rules/no-side-effects.md) | Warns about unexpected side effects in computed properties |
 |  | [require-return-from-computed](./docs/rules/require-return-from-computed.md) | Warns about missing return statements in computed properties |
 | :white_check_mark: | [require-super-in-init](./docs/rules/require-super-in-init.md) | Enforces super calls in init hooks |

docs/rules/no-leaking-state-in-program-scope.md

@@ -0,0 +1,54 @@
+# Do not allow anything else than constants in global module namespace. (no-leaking-state-in-program-scope)
+
+This rule attempts to catch any possible hidden state thanks to program scope variables.
+
+
+## Rule Details
+
+Program scope variables that can store value retrieved at runtime can potentially introduce bugs in tests where they might pollute environments between two tests.
+
+Consider following service:
+
+```js
+var cachedFoo = null;
+
+export default Service.extend({
+  fetchFoo(count) {
+    if(!cachedFoo) {
+      fetch(`./api/?results=${count}`).then(function(response) {
+        cachedFoo = response;
+      });
+    }
+  }
+
+  getFoo() {
+    return cachedFoo;
+  }
+});
+```
+
+If there are two tests calling this service with different value for `count` when calling `fetchFoo`, then calls to `getFoo` will always return the response of the first test.
+
+Examples of **incorrect** code for this rule:
+
+```js
+var foo = 'bar';
+
+export default Service.extend({});
+```
+
+Examples of **correct** code for this rule:
+
+```js
+const foo = 'bar';
+
+export default Service.extend({});
+```
+
+```js
+export default Service.extend({
+  init() {
+    this.set('foo', 'bar');
+  }
+});
+```

lib/recommended-rules.js

@@ -31,6 +31,7 @@ module.exports = {
   "ember/no-global-jquery": "error",
   "ember/no-invalid-debug-function-arguments": "off",
   "ember/no-jquery": "off",
+  "ember/no-leaking-state-in-program-scope": "off",
   "ember/no-new-mixins": "off",
   "ember/no-observers": "error",
   "ember/no-old-shims": "error",

lib/rules/no-leaking-state-in-program-scope.js

@@ -0,0 +1,28 @@
+'use strict';
+
+const message = 'Use const on global scope';
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Do not allow anything else than constants in global module namespace.',
+      category: 'Possible Errors',
+      recommended: false,
+    },
+    fixable: null,
+  },
+
+  create(context) {
+    const report = function(node) {
+      context.report(node, message);
+    };
+
+    return {
+      VariableDeclaration(node) {
+        if (node.parent.type === 'Program' && node.kind !== 'const') {
+          report(node, message);
+        }
+      },
+    };
+  },
+};

tests/lib/rules/no-leaking-state-in-module-scope.js

@@ -0,0 +1,41 @@
+'use strict';
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/no-leaking-state-in-program-scope');
+const RuleTester = require('eslint').RuleTester;
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+let ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 2018,
+    sourceType: 'module',
+  },
+});
+
+ruleTester.run('no-leaking-state-in-program-scope', rule, {
+  valid: [
+    `const foo = "bar";`,
+    `export default Ember.Service.extend({
+      init() {
+        var foo = "bar";
+      }
+    });`,
+  ],
+
+  invalid: [
+    {
+      code: `var foo = "bar";`,
+      errors: [{ message: 'Use const on global scope', type: 'VariableDeclaration' }],
+    },
+    {
+      code: `let foo = "bar";`,
+      errors: [{ message: 'Use const on global scope', type: 'VariableDeclaration' }],
+    },
+  ],
+});