Add "require-default-prop" rule

Author: michalsnik

docs/rules/require-default-prop.md

@@ -0,0 +1,45 @@
+# Require default value for props (require-default-prop)
+
+This rule requires default value to be set for each props that are not marked as `required`.
+
+## Rule Details
+
+Examples of **incorrect** code for this rule:
+
+```js
+export default {
+  props: {
+    a: Number,
+    b: [Number, String],
+    c: {
+      type: Number
+    },
+    d: {
+      type: Number,
+      required: false
+    }
+  }
+}
+```
+
+Examples of **correct** code for this rule:
+
+```js
+export default {
+  props: {
+    a: {
+      type: Number,
+      required: true
+    },
+    b: {
+      type: Number,
+      default: 0
+    },
+    c: {
+      type: Number,
+      default: 0,
+      required: false
+    }
+  }
+}
+```

lib/rules/require-default-prop.js

@@ -0,0 +1,98 @@
+/**
+ * @fileoverview Require default value for props
+ * @author Michał Sajnóg <[email protected]> (http://github.com/michalsnik)
+ */
+'use strict'
+
+const utils = require('../utils')
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    docs: {
+      description: 'Require default value for props',
+      category: 'Best Practices',
+      recommended: false
+    },
+    fixable: null,  // or "code" or "whitespace"
+    schema: []
+  },
+
+  create: function (context) {
+    // ----------------------------------------------------------------------
+    // Helpers
+    // ----------------------------------------------------------------------
+
+    /**
+     * Checks if the passed prop is required
+     * @param {Property} prop - Property AST node for a single prop
+     * @return {boolean}
+     */
+    function propIsRequired (prop) {
+      const propRequiredNode = prop.value.properties
+        .find(p =>
+          p.key.name === 'required' &&
+          p.value.type === 'Literal' &&
+          p.value.value === true
+        )
+
+      return Boolean(propRequiredNode)
+    }
+
+    /**
+     * Checks if the passed prop has a defualt value
+     * @param {Property} prop - Property AST node for a single prop
+     * @return {boolean}
+     */
+    function propHasDefault (prop) {
+      const propDefaultNode = prop.value.properties
+        .find(p => p.key.name === 'default')
+
+      return Boolean(propDefaultNode)
+    }
+
+    /**
+     * Finds all props that don't have a default value set
+     * @param {Property} propsNode - Vue component's "props" node
+     * @return {boolean}
+     */
+    function findPropsWithoutDefaultValue (propsNode) {
+      return propsNode.value.properties
+        .filter(prop => {
+          if (prop.value.type !== 'ObjectExpression') return true
+
+          return !propIsRequired(prop) && !propHasDefault(prop)
+        })
+    }
+
+    // ----------------------------------------------------------------------
+    // Public
+    // ----------------------------------------------------------------------
+
+    return utils.executeOnVue(context, (obj) => {
+      const propsNode = obj.properties
+        .find(p =>
+          p.type === 'Property' &&
+          p.key.type === 'Identifier' &&
+          p.key.name === 'props'
+        )
+
+      if (!propsNode || propsNode.value.type !== 'ObjectExpression') return
+
+      const propsWithoutDefault = findPropsWithoutDefaultValue(propsNode)
+
+      propsWithoutDefault.forEach(prop => {
+        context.report({
+          node: prop,
+          message: `Prop '{{propName}}' requires default value to be set.`,
+          data: {
+            propName: prop.key.name
+          }
+        })
+      })
+    })
+  }
+}

tests/lib/rules/require-default-prop.js

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+/**
+ * @fileoverview Require default value for props
+ * @author Michał Sajnóg <[email protected]> (http://github.com/michalsnik)
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/require-default-prop')
+const RuleTester = require('eslint').RuleTester
+const parserOptions = { ecmaVersion: 6, sourceType: 'module' }
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester()
+ruleTester.run('require-default-prop', rule, {
+
+  valid: [
+    {
+      filename: 'test.vue',
+      code: `
+        export default {
+          props: {
+            a: {
+              type: Number,
+              required: true
+            },
+            b: {
+              type: Number,
+              default: 0
+            },
+            c: {
+              type: Number,
+              default: 0,
+              required: false
+            },
+            // eslint-disable-next-line require-default-prop
+            d: Number
+          }
+        }
+      `,
+      parserOptions
+    }
+  ],
+
+  invalid: [
+    {
+      filename: 'test.vue',
+      code: `
+        export default {
+          props: {
+            a: Number,
+            b: [Number, String],
+            c: {
+              type: Number
+            },
+            d: {
+              type: Number,
+              required: false
+            }
+          }
+        }
+      `,
+      parserOptions,
+      errors: [{
+        message: `Prop 'a' requires default value to be set.`,
+        line: 4
+      }, {
+        message: `Prop 'b' requires default value to be set.`,
+        line: 5
+      }, {
+        message: `Prop 'c' requires default value to be set.`,
+        line: 6
+      }, {
+        message: `Prop 'd' requires default value to be set.`,
+        line: 9
+      }]
+    }
+  ]
+})