Add new vue/no-arrow-functions-in-watch rule

docs/rules/README.md

@@ -286,6 +286,7 @@ For example:
 | [vue/keyword-spacing](./keyword-spacing.md) | enforce consistent spacing before and after keywords | :wrench: |
 | [vue/match-component-file-name](./match-component-file-name.md) | require component name property to match its file name |  |
 | [vue/max-len](./max-len.md) | enforce a maximum line length |  |
+| [vue/no-arrow-functions-in-watch](./no-arrow-functions-in-watch.md)| disallows using arrow functions to define wathcer |  |
 | [vue/no-boolean-default](./no-boolean-default.md) | disallow boolean defaults | :wrench: |
 | [vue/no-duplicate-attr-inheritance](./no-duplicate-attr-inheritance.md) | enforce `inheritAttrs` to be set to `false` when using `v-bind="$attrs"` |  |
 | [vue/no-empty-pattern](./no-empty-pattern.md) | disallow empty destructuring patterns |  |

docs/rules/no-arrow-functions-in-watch.md

@@ -0,0 +1,61 @@
+---
+pageClass: rule-details
+sidebarDepth: 0
+title: vue/no-arrow-functions-in-watch
+description: disallow arrow functions to define watcher
+---
+# vue/no-arrow-functions-in-watch
+> disallow using arrow functions to define watcher
+
+## :book: Rule Details
+
+This rules disallows using arrow functions to defined watcher.The reason is arrow functions bind the parent context, so `this` will not be the Vue instance as you expect.([see here for more details](https://vuejs.org/v2/api/#watch))
+
+<eslint-code-block :rules="{'vue/no-arrow-functions-in-watch': ['error']}">
+
+```vue
+<script>
+export default {
+  watch: {
+    /* ✓ GOOD */
+    a: function (val, oldVal) {
+      console.log('new: %s, old: %s', val, oldVal)
+    },
+    b: 'someMethod',
+    c: {
+      handler: function (val, oldVal) { /* ... */ },
+      deep: true
+    },
+    d: {
+      handler: 'someMethod',
+      immediate: true
+    },
+    e: [
+      'handle1',
+      function handle2 (val, oldVal) { /* ... */ },
+      {
+        handler: function handle3 (val, oldVal) { /* ... */ },
+        /* ... */
+      }
+    ],
+    'e.f': function (val, oldVal) { /* ... */ }
+
+    /* ✗ BAD */
+    foo: (val, oldVal) => {
+      console.log('new: %s, old: %s', val, oldVal)
+    }
+  }
+}
+</script>
+```
+
+</eslint-code-block>
+
+## :wrench: Options
+
+Nothing.
+
+## :mag: Implementation
+
+- [Rule source](https://github.com/vuejs/eslint-plugin-vue/blob/master/lib/rules/no-arrow-functions-in-watch.js)
+- [Test source](https://github.com/vuejs/eslint-plugin-vue/blob/master/tests/lib/rules/no-arrow-functions-in-watch.js)

lib/index.js

@@ -40,6 +40,7 @@ module.exports = {
     'multiline-html-element-content-newline': require('./rules/multiline-html-element-content-newline'),
     'mustache-interpolation-spacing': require('./rules/mustache-interpolation-spacing'),
     'name-property-casing': require('./rules/name-property-casing'),
+    'no-arrow-functions-in-watch': require('./rules/no-arrow-functions-in-watch'),
     'no-async-in-computed-properties': require('./rules/no-async-in-computed-properties'),
     'no-boolean-default': require('./rules/no-boolean-default'),
     'no-confusing-v-for-v-if': require('./rules/no-confusing-v-for-v-if'),

lib/rules/no-arrow-functions-in-watch.js

@@ -0,0 +1,39 @@
+/**
+ * @author Sosuke Suzuki
+ */
+'use strict'
+
+const utils = require('../utils')
+
+module.exports = {
+  meta: {
+    type: 'problem',
+    docs: {
+      description: 'disallow using arrow functions to define watcher',
+      categories: undefined,
+      url: 'https://eslint.vuejs.org/rules/no-arrow-functions-in-watch.html'
+    },
+    fixable: null,
+    schema: []
+  },
+  create (context) {
+    return utils.executeOnVue(context, (obj) => {
+      const watchNode = obj.properties.find((property) => utils.getStaticPropertyName(property) === 'watch')
+      if (watchNode == null) {
+        return
+      }
+      const watchValue = watchNode.value
+      if (watchValue.type !== 'ObjectExpression') {
+        return
+      }
+      for (const property of watchValue.properties) {
+        if (property.value.type === 'ArrowFunctionExpression') {
+          context.report({
+            node: property,
+            message: 'You should not use an arrow function to define a watcher.'
+          })
+        }
+      }
+    })
+  }
+}

tests/lib/rules/no-arrow-functions-in-watch.js

@@ -0,0 +1,196 @@
+/**
+ * @author Sosuke Suzuki
+ */
+
+// ------------------------------------------------------------------------------
+// Requirements
+// ------------------------------------------------------------------------------
+
+const rule = require('../../../lib/rules/no-arrow-functions-in-watch')
+const RuleTester = require('eslint').RuleTester
+
+// ------------------------------------------------------------------------------
+// Tests
+// ------------------------------------------------------------------------------
+
+const ruleTester = new RuleTester({
+  parserOptions: {
+    ecmaVersion: 2018,
+    sourceType: 'module'
+  }
+})
+ruleTester.run('no-arrow-functions-in-watch', rule, {
+  valid: [
+    {
+      filename: 'test.vue',
+      code: `
+        export default {}
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+        export default {
+          watch: {}
+        }
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+        export default {
+          watch: {
+            foo() {}
+          },
+        }
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+        export default {
+          watch: {
+            foo: function() {}
+          },
+        }
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+        export default {
+          watch: {
+            foo() {},
+            bar() {}
+          },
+        }
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+        export default {
+          watch: {
+            foo: function() {},
+            bar: function() {}
+          },
+        }
+      `
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      export default {
+        data: {
+          a: 1,
+          b: 2,
+          c: 3,
+          d: 4,
+          e: {
+            f: {
+              g: 5
+            }
+          }
+        },
+        watch: {
+          a: function (val, oldVal) {
+            console.log('new: %s, old: %s', val, oldVal)
+          },
+          b: 'someMethod',
+          c: {
+            handler: function (val, oldVal) {},
+            deep: true
+          },
+          d: {
+            handler: 'someMethod',
+            immediate: true
+          },
+          e: [
+            'handle1',
+            function handle2 (val, oldVal) {},
+            {
+              handler: function handle3 (val, oldVal) {},
+              /* ... */
+            }
+          ],
+          'e.f': function (val, oldVal) { /* ... */ }
+        }
+      }`
+    }
+  ],
+  invalid: [
+    {
+      filename: 'test.vue',
+      code: `
+      export default {
+        watch: {
+          foo: () => {}
+        },
+      }`,
+      errors: ['You should not use an arrow function to define a watcher.']
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      export default {
+        watch: {
+          foo() {},
+          bar: () => {}
+        }
+      }`,
+      errors: ['You should not use an arrow function to define a watcher.']
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      export default {
+        watch: {
+          foo: function() {},
+          bar: () => {}
+        }
+      }`,
+      errors: ['You should not use an arrow function to define a watcher.']
+    },
+    {
+      filename: 'test.vue',
+      code: `
+      export default {
+        data: {
+          a: 1,
+          b: 2,
+          c: 3,
+          d: 4,
+          e: {
+            f: {
+              g: 5
+            }
+          }
+        },
+        watch: {
+          a: (val, oldVal) => {
+            console.log('new: %s, old: %s', val, oldVal)
+          },
+          b: 'someMethod',
+          c: {
+            handler: function (val, oldVal) {},
+            deep: true
+          },
+          d: {
+            handler: 'someMethod',
+            immediate: true
+          },
+          e: [
+            'handle1',
+            function handle2 (val, oldVal) {},
+            {
+              handler: function handle3 (val, oldVal) {},
+              /* ... */
+            }
+          ],
+          'e.f': function (val, oldVal) { /* ... */ }
+        }
+      }`,
+      errors: ['You should not use an arrow function to define a watcher.']
+    }
+  ]
+})