fix vuejs#255: Add new rule require-v-if-else-key

Author: doug-wade

docs/rules/README.md

@@ -12,6 +12,7 @@ sidebarDepth: 0
   :bulb: Indicates that some problems reported by the rule are manually fixable by editor [suggestions](https://eslint.org/docs/developer-guide/working-with-rules#providing-suggestions).
 :::
 
+
 ## Base Rules (Enabling Correct ESLint Parsing)
 
 Enforce all the rules in this category, as well as all higher priority rules, with:
@@ -361,6 +362,7 @@ For example:
 | [vue/require-emit-validator](./require-emit-validator.md) | require type definitions in emits | :bulb: |
 | [vue/require-expose](./require-expose.md) | require declare public properties using `expose` | :bulb: |
 | [vue/require-name-property](./require-name-property.md) | require a name property in Vue components |  |
+| [vue/require-v-if-else-key](./require-v-if-else-key.md) | require `key` with `v-if/v-else-if/v-else` directives |  |
 | [vue/script-indent](./script-indent.md) | enforce consistent indentation in `<script>` | :wrench: |
 | [vue/sort-keys](./sort-keys.md) | enforce sort-keys in a manner that is compatible with order-in-components |  |
 | [vue/static-class-names-order](./static-class-names-order.md) | enforce static class names order | :wrench: |

docs/rules/require-v-if-else-key.md

@@ -0,0 +1,52 @@
+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/require-v-if-else-key
+description: require `key` with `v-if/v-else-if/v-else` directives
+---
+# vue/require-v-if-else-key
+
+> require `key` with `v-if/v-else-if/v-else` directives
+
+- :exclamation: <badge text="This rule has not been released yet." vertical="middle" type="error"> ***This rule has not been released yet.*** </badge>
+
+## :book: Rule Details
+
+This rule reports the elements which have `v-if`, `v-else-if`, and/or `v-else` and do not have a `v-bind:key` or `key`.
+
+<eslint-code-block :rules="{'vue/require-v-if-else-key': ['error']}">
+
+```vue
+<template>
+  <!-- ✓ GOOD -->
+  <div v-if="condition"/>
+
+  <div v-if="condition"/>
+  <span v-else>
+
+  <div v-if="condition" key="one"/>
+  <div v-else key="two"/>
+
+  <div v-if="condition" :key="one"/>
+  <div v-else :key="two"/>
+
+  <!-- ✗ BAD -->
+  <div v-if="condition"/>
+  <div v-else/>
+</template>
+```
+
+</eslint-code-block>
+
+## :wrench: Options
+
+Nothing.
+
+## :books: Further Reading
+
+- [Guide (for v2) - v-if without key](https://v2.vuejs.org/v2/style-guide/?redirect=true#v-if-v-else-if-v-else-without-key-use-with-caution)
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/require-v-if-else-key.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/require-v-if-else-key.js)

lib/index.js

@@ -179,6 +179,7 @@ module.exports = {
     'require-slots-as-functions': require('./rules/require-slots-as-functions'),
     'require-toggle-inside-transition': require('./rules/require-toggle-inside-transition'),
     'require-v-for-key': require('./rules/require-v-for-key'),
+    'require-v-if-else-key': require('./rules/require-v-if-else-key'),
     'require-valid-default-prop': require('./rules/require-valid-default-prop'),
     'return-in-computed-property': require('./rules/return-in-computed-property'),
     'return-in-emits-validator': require('./rules/return-in-emits-validator'),

lib/rules/require-v-if-else-key.js

@@ -0,0 +1,102 @@
+/**
+ * @author Doug Wade <[email protected]>
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const utils = require('../utils')
+
+// ------------------------------------------------------------------------------
+// Helpers
+// ------------------------------------------------------------------------------
+
+// ...
+
+// ------------------------------------------------------------------------------
+// Rule Definition
+// ------------------------------------------------------------------------------
+
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'require `key` with `v-if/v-else-if/v-else` directives',
+      categories: undefined,
+      url: 'https://eslint.vuejs.org/rules/require-v-if-else-key.html'
+    },
+    fixable: null,
+    schema: [],
+    messages: {
+      unexpected:
+        'Elements in v-if/v-else-if/v-else expect to have distinct keys if they are of the same type.'
+    }
+  },
+  /** @param {RuleContext} context */
+  create(context) {
+    const NO_MATCH = Symbol('no match')
+
+    /**
+     * Gets the key from an element.
+     * @param {VElement} element The element node to get the key from.
+     * @return {String | null}
+     */
+    function getKey(element) {
+      const keyAttribute = utils.getAttribute(element, 'key')
+      if (keyAttribute) {
+        return keyAttribute?.value?.value || ''
+      }
+
+      const keyDirective = utils.getDirective(element, 'bind', 'key')
+      if (!keyDirective) {
+        return ''
+      }
+
+      if (keyDirective.value?.expression?.type !== 'Identifier') {
+        return null
+      }
+
+      return `v-bind:${keyDirective.value?.expression?.name}`
+    }
+
+    /**
+     * Check the given element about `v-bind:key` attributes.
+     * @param {VElement} element The element node to check.
+     */
+    function checkKey(element) {
+      const prevSibling = utils.prevSibling(element)
+
+      if (!prevSibling) {
+        return
+      }
+
+      if (prevSibling.name !== element.name) {
+        return
+      }
+
+      const key = getKey(element)
+
+      if (key === null) {
+        return
+      }
+
+      if (!key || key === getKey(prevSibling)) {
+        context.report({ node: element, messageId: 'unexpected' })
+      }
+    }
+
+    return utils.defineTemplateBodyVisitor(context, {
+      /** @param {VDirective} node */
+      "VAttribute[directive=true][key.name.name='else']"(node) {
+        checkKey(node.parent.parent)
+      },
+      /** @param {VDirective} node */
+      "VAttribute[directive=true][key.name.name='else-if']"(node) {
+        checkKey(node.parent.parent)
+      }
+    })
+  }
+}

tests/lib/rules/require-v-if-else-key.js

@@ -0,0 +1,90 @@
+/**
+ * @author Doug Wade <[email protected]>
+ * See LICENSE file in root directory for full license.
+ */
+'use strict'
+
+const RuleTester = require('eslint').RuleTester
+const rule = require('../../../lib/rules/require-v-if-else-key')
+
+const tester = new RuleTester({
+  parser: require.resolve('vue-eslint-parser'),
+  parserOptions: {
+    ecmaVersion: 2020,
+    sourceType: 'module'
+  }
+})
+
+tester.run('require-v-if-else-key', rule, {
+  valid: [
+    {
+      filename: 'test.vue',
+      code: `
+      <template>
+        <div>
+          <div v-if="conditional" :key="foo">foo</div>
+          <div v-else :key="bar">bar</div>
+        </div>
+      </template>
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <template>
+        <div>
+          <div v-if="conditional" key="foo">foo</div>
+          <div v-else key="bar">bar</div>
+        </div>
+      </template>
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <template>
+        <div>
+          <div v-if="conditional">foo</div>
+        </div>
+      </template>
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <template>
+        <div>
+          <div v-if="conditional">foo</div>
+          <span v-else>bar</span>
+        </div>
+      </template>
+      `
+    }
+  ],
+  invalid: [
+    {
+      filename: 'test.vue',
+      code: `
+      <template>
+        <div v-if="conditional">foo</div>
+        <div v-else>bar</div>
+      </template>
+      `,
+      errors: [
+        'Elements in v-if/v-else-if/v-else expect to have distinct keys if they are of the same type.'
+      ]
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      <template>
+        <div v-if="conditional" key="foo">foo</div>
+        <div v-else key="foo">bar</div>
+      </template>
+      `,
+      errors: [
+        'Elements in v-if/v-else-if/v-else expect to have distinct keys if they are of the same type.'
+      ]
+    }
+  ]
+})