Add rule vue/require-valid-default-prop. (#119)

* Add rule `vue/require-valid-default-prop`.
fixes #117

* Update doc & error message & add more tests

Author: armano2

Diff for: docs/rules/require-valid-default-prop.md

@@ -0,0 +1,63 @@
+# Enforces props default values to be valid (require-valid-default-prop)
+
+This rule checks whether the default value of each prop is valid for the given type. It should report an error when default value for type `Array` or `Object` is not returned using function.
+
+## :book: Rule Details
+
+:-1: Examples of **incorrect** code for this rule:
+
+```js
+Vue.component('example', {
+  props: {
+    propA: {
+      type: String,
+      default: {}
+    },
+    propB: {
+      type: String,
+      default: []
+    },
+    propC: {
+      type: Object,
+      default: []
+    },
+    propD: {
+      type: Array,
+      default: []
+    },
+    propE: {
+      type: Object,
+      default: { message: 'hello' }
+    }
+  }
+})
+```
+
+:+1: Examples of **correct** code for this rule:
+
+```js
+Vue.component('example', {
+  props: {
+    // basic type check (`null` means accept any type)
+    propA: Number,
+    // multiple possible types
+    propB: [String, Number],
+    // a number with default value
+    propD: {
+      type: Number,
+      default: 100
+    },
+    // object/array defaults should be returned from a factory function
+    propE: {
+      type: Object,
+      default: function () {
+        return { message: 'hello' }
+      }
+    }
+  }
+})
+```
+
+## :wrench: Options
+
+Nothing.

Diff for: lib/rules/require-valid-default-prop.js

@@ -0,0 +1,134 @@
+/**
+ * @fileoverview Enforces props default values to be valid.
+ * @author Armano
+ */
+'use strict'
+const utils = require('../utils')
+
+const NATIVE_TYPES = new Set([
+  'String',
+  'Number',
+  'Boolean',
+  'Function',
+  'Object',
+  'Array',
+  'Symbol'
+])
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Enforces props default values to be valid.',
+      category: 'Possible Errors',
+      recommended: false
+    },
+    fixable: null,
+    schema: []
+  },
+
+  create (context) {
+    // ----------------------------------------------------------------------
+    // Helpers
+    // ----------------------------------------------------------------------
+
+    function isPropertyIdentifier (node) {
+      return node.type === 'Property' && node.key.type === 'Identifier'
+    }
+
+    function getPropertyNode (obj, name) {
+      return obj.properties.find(p =>
+        isPropertyIdentifier(p) &&
+        p.key.name === name
+      )
+    }
+
+    function getTypes (node) {
+      if (node.type === 'Identifier') {
+        return [node.name]
+      } else if (node.type === 'ArrayExpression') {
+        return node.elements
+          .filter(item => item.type === 'Identifier')
+          .map(item => item.name)
+      }
+      return []
+    }
+
+    function ucFirst (text) {
+      return text[0].toUpperCase() + text.slice(1)
+    }
+
+    function getValueType (node) {
+      if (node.type === 'CallExpression') { // Symbol(), Number() ...
+        if (node.callee.type === 'Identifier' && NATIVE_TYPES.has(node.callee.name)) {
+          return node.callee.name
+        }
+      } else if (node.type === 'TemplateLiteral') { // String
+        return 'String'
+      } else if (node.type === 'Literal') { // String, Boolean, Number
+        if (node.value === null) return null
+        const type = ucFirst(typeof node.value)
+        if (NATIVE_TYPES.has(type)) {
+          return type
+        }
+      } else if (node.type === 'ArrayExpression') { // Array
+        return 'Array'
+      } else if (node.type === 'ObjectExpression') { // Object
+        return 'Object'
+      }
+      // FunctionExpression, ArrowFunctionExpression
+      return null
+    }
+
+    // ----------------------------------------------------------------------
+    // Public
+    // ----------------------------------------------------------------------
+
+    return utils.executeOnVue(context, obj => {
+      const props = obj.properties.find(p =>
+        isPropertyIdentifier(p) &&
+        p.key.name === 'props' &&
+        p.value.type === 'ObjectExpression'
+      )
+      if (!props) return
+
+      const properties = props.value.properties.filter(p =>
+        isPropertyIdentifier(p) &&
+        p.value.type === 'ObjectExpression'
+      )
+
+      for (const prop of properties) {
+        const type = getPropertyNode(prop.value, 'type')
+        if (!type) {
+          return
+        }
+
+        const typeNames = new Set(getTypes(type.value)
+          .map(item => item === 'Object' || item === 'Array' ? 'Function' : item) // Object and Array require function
+          .filter(item => NATIVE_TYPES.has(item)))
+
+        if (typeNames.size === 0) { // There is no native types detected
+          return
+        }
+
+        const def = getPropertyNode(prop.value, 'default')
+        if (!def) return
+
+        const defType = getValueType(def.value)
+        if (typeNames.has(defType)) return
+
+        context.report({
+          node: def,
+          message: "Type of the default value for '{{name}}' prop must be a {{types}}.",
+          data: {
+            name: prop.key.name,
+            types: Array.from(typeNames).join(' or ').toLowerCase()
+          }
+        })
+      }
+    })
+  }
+}