Adding return-from-computed rule. Fixes ember-cli#247.

docs/rules/return-from-computed.md

@@ -0,0 +1,44 @@
+## Always return a value from a computed property function
+
+### Rule name: `return-from-computed`
+
+When using computed properties always return a value.
+
+```javascript
+import Ember from 'ember';
+
+const {
+  Component,
+  computed
+} = Ember;
+
+export default Component.extend({
+  firstName: null,
+  lastName: null,
+
+  // GOOD
+  fullName: computed('firstName', 'lastName', {
+    get(key) {
+      return `${this.get('firstName')} ${this.get('lastName')}`;
+    },
+    set(key, value) {
+      let [firstName, lastName] = value.split(/\s+/);
+      this.set('firstName', firstName);
+      this.set('lastName',  lastName);
+      return value;
+    }
+  }),
+
+  // BAD
+  fullName: computed('firstName', 'lastName', {
+    get(key) {
+      return `${this.get('firstName')} ${this.get('lastName')}`;
+    },
+    set(key, value) {
+      let [firstName, lastName] = value.split(/\s+/);
+      this.set('firstName', firstName);
+      this.set('lastName',  lastName);
+    }
+  })
+});
+```

lib/index.js

@@ -31,6 +31,7 @@ module.exports = {
     'order-in-controllers': require('./rules/order-in-controllers'),
     'order-in-models': require('./rules/order-in-models'),
     'order-in-routes': require('./rules/order-in-routes'),
+    'return-from-computed': require('./rules/return-from-computed'),
     'require-super-in-init': require('./rules/require-super-in-init'),
     'routes-segments-snake-case': require('./rules/routes-segments-snake-case'),
     'use-brace-expansion': require('./rules/use-brace-expansion'),

lib/rules/return-from-computed.js

@@ -0,0 +1,81 @@
+'use strict';
+
+
+const { isComputedProp } = require('../utils/ember');
+
+//------------------------------------------------------------------------------
+// General rule - Don't introduce side-effects in computed properties
+//------------------------------------------------------------------------------
+
+function isReachable(segment) {
+  return segment.reachable;
+}
+
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Warns about missing return statements in computed properties',
+      category: 'Possible Errors',
+      recommended: false,
+      url: 'https://github.com/ember-cli/eslint-plugin-ember/tree/master/docs/rules/no-side-effects.md'
+    },
+    fixable: null, // or "code" or "whitespace"
+  },
+
+  create(context) {
+    const message = 'Always return a value from computed properties';
+
+    const report = function (node) {
+      context.report(node, message);
+    };
+
+    let funcInfo = {
+      upper: null,
+      codePath: null,
+      hasReturn: false,
+      shouldCheck: false,
+      node: null
+    };
+
+    function checkLastSegment(node) {
+      if (funcInfo.shouldCheck &&
+          funcInfo.codePath.currentSegments.some(isReachable)
+      ) {
+        report(node);
+      }
+    }
+
+    return {
+      onCodePathStart(codePath, node) {
+        funcInfo = {
+          upper: funcInfo,
+          codePath,
+          hasReturn: false,
+          shouldCheck: context.getAncestors().findIndex(isComputedProp) > -1,
+          node
+        };
+      },
+
+      onCodePathEnd() {
+        funcInfo = funcInfo.upper;
+      },
+
+      ReturnStatement(node) {
+        if (funcInfo.shouldCheck) {
+          funcInfo.hasReturn = true;
+
+          if (!node.argument) {
+            report(node);
+          }
+        }
+      },
+
+      'FunctionExpression:exit': function (node) {
+        if (isComputedProp(node.parent) || isComputedProp(node.parent.parent.parent)) {
+          checkLastSegment(node);
+        }
+      }
+    };
+  }
+};

tests/lib/rules/return-from-computed.js

@@ -0,0 +1,78 @@
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/return-from-computed');
+const RuleTester = require('eslint').RuleTester;
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+const eslintTester = new RuleTester();
+eslintTester.run('return-from-computed', rule, {
+  valid: [
+    {
+      code: 'let foo = computed("test", function() { return ""; })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+    },
+    {
+      code: 'let foo = computed("test", { get() { return true; }, set() { return true; } })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+    },
+    {
+      code: 'let foo = computed("test", function() { if (true) { return ""; } return ""; })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+    },
+    {
+      code: 'let foo = computed("test", { get() { data.forEach(function() { }); return true; }, set() { return true; } })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+    },
+    {
+      code: 'let foo = computed("test", function() { data.forEach(function() { }); return ""; })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+    }
+  ],
+  invalid: [
+    {
+      code: 'let foo = computed("test", function() { })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+      errors: [{
+        message: 'Always return a value from computed properties'
+      }]
+    },
+    {
+      code: 'let foo = computed("test", function() { return; })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+      errors: [{
+        message: 'Always return a value from computed properties'
+      }]
+    },
+    {
+      code: 'let foo = computed("test", function() { if (true) { return ""; } })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+      errors: [{
+        message: 'Always return a value from computed properties'
+      }]
+    },
+    {
+      code: 'let foo = computed("test", { get() {}, set() {} })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+      errors: [
+        {
+          message: 'Always return a value from computed properties'
+        },
+        {
+          message: 'Always return a value from computed properties'
+        }
+      ]
+    },
+    {
+      code: 'let foo = computed("test", function() { })',
+      parserOptions: { ecmaVersion: 6, sourceType: 'module' },
+      errors: [{
+        message: 'Always return a value from computed properties'
+      }]
+    }
+  ],
+});