Add no-side-effects-in-computed-properties rule

Author: michalsnik

docs/rules/no-side-effects-in-computed-properties.md

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+# Do not introduce side effects in computed properties (no-side-effects-in-computed-properties)
+
+It is considered a very bad practice to introduce side effects inside computed properties. It makes the code not predictable and hard to understand.
+
+
+## Rule Details
+
+Examples of **incorrect** code for this rule:
+
+```js
+
+export default {
+  computed: {
+    fullName() {
+      this.firstName = 'lorem' // <- side effect
+      return `${this.firstName} ${this.lastName}`
+    },
+    somethingReversed() {
+      return this.something.reverse() // <- side effect
+    }
+  }
+}
+
+```
+
+Examples of **correct** code for this rule:
+
+```js
+
+export default {
+  computed: {
+    fullName() {
+      return `${this.firstName} ${this.lastName}`
+    },
+    somethingReversed() {
+      return this.something.slice(0).reverse()
+    }
+  }
+}
+
+```

lib/rules/no-side-effects-in-computed-properties.js

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+/**
+ * @fileoverview Don't introduce side effects in computed properties
+ * @author Michał Sajnóg
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Rule Definition
+//------------------------------------------------------------------------------
+
+module.exports = {
+    meta: {
+        docs: {
+            description: "Don't introduce side effects in computed properties",
+            category: "Fill me in",
+            recommended: false
+        },
+        fixable: null,  // or "code" or "whitespace"
+        schema: [
+            // fill in your schema
+        ]
+    },
+
+    create: function(context) {
+
+        // variables should be defined here
+
+        //----------------------------------------------------------------------
+        // Helpers
+        //----------------------------------------------------------------------
+
+        // any helper functions should go here or else delete this section
+
+        //----------------------------------------------------------------------
+        // Public
+        //----------------------------------------------------------------------
+
+        return {
+
+            // give me methods
+
+        };
+    }
+};

tests/lib/rules/no-side-effects-in-computed-properties.js

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+/**
+ * @fileoverview Don't introduce side effects in computed properties
+ * @author Michał Sajnóg
+ */
+"use strict";
+
+//------------------------------------------------------------------------------
+// Requirements
+//------------------------------------------------------------------------------
+
+var rule = require("../../../lib/rules/no-side-effects-in-computed-properties"),
+
+    RuleTester = require("eslint").RuleTester;
+
+
+//------------------------------------------------------------------------------
+// Tests
+//------------------------------------------------------------------------------
+
+var ruleTester = new RuleTester();
+ruleTester.run("no-side-effects-in-computed-properties", rule, {
+
+    valid: [
+
+        // give me some code that won't trigger a warning
+    ],
+
+    invalid: [
+        {
+            code: "",
+            errors: [{
+                message: "Fill me in.",
+                type: "Me too"
+            }]
+        }
+    ]
+});