feat: add require-compound-type-alias (#365)

.README/rules/require-compound-type-alias.md

@@ -0,0 +1,14 @@
+### `require-compound-type-alias`
+
+Requires to make a type alias for all [union](https://flow.org/en/docs/types/unions/) and [intersection](https://flow.org/en/docs/types/intersections/) types. If these are used in "raw" forms it might be tempting to just copy&paste them around the code. However, this brings sort of a source code pollution and unnecessary changes on several parts when these compound types need to be changed.
+
+#### Options
+
+The rule has a string option:
+
+* `"never"`
+* `"always"`
+
+The default value is `"always"`.
+
+<!-- assertions require-compound-type-alias -->

src/index.js

@@ -16,6 +16,7 @@ import noTypesMissingFileAnnotation from './rules/noTypesMissingFileAnnotation';
 import noUnusedExpressions from './rules/noUnusedExpressions';
 import noWeakTypes from './rules/noWeakTypes';
 import objectTypeDelimiter from './rules/objectTypeDelimiter';
+import requireCompoundTypeAlias from './rules/requireCompoundTypeAlias';
 import requireExactType from './rules/requireExactType';
 import requireParameterType from './rules/requireParameterType';
 import requireReturnType from './rules/requireReturnType';
@@ -51,6 +52,7 @@ const rules = {
   'no-unused-expressions': noUnusedExpressions,
   'no-weak-types': noWeakTypes,
   'object-type-delimiter': objectTypeDelimiter,
+  'require-compound-type-alias': requireCompoundTypeAlias,
   'require-exact-type': requireExactType,
   'require-parameter-type': requireParameterType,
   'require-return-type': requireReturnType,
@@ -94,6 +96,7 @@ export default {
     'no-mutable-array': 0,
     'no-weak-types': 0,
     'object-type-delimiter': 0,
+    'require-compound-type-alias': 0,
     'require-exact-type': 0,
     'require-parameter-type': 0,
     'require-return-type': 0,

src/rules/requireCompoundTypeAlias.js

@@ -0,0 +1,38 @@
+const schema = [
+  {
+    enum: ['always', 'never'],
+    type: 'string'
+  }
+];
+
+const create = (context) => {
+  const always = (context.options[0] || 'always') === 'always';
+
+  if (always) {
+    return {
+      IntersectionTypeAnnotation (node) {
+        if (node.parent.type !== 'TypeAlias') {
+          context.report({
+            message: 'All intersection types must be declared with named type alias.',
+            node
+          });
+        }
+      },
+      UnionTypeAnnotation (node) {
+        if (node.parent.type !== 'TypeAlias') {
+          context.report({
+            message: 'All union types must be declared with named type alias.',
+            node
+          });
+        }
+      }
+    };
+  } else {
+    return {};
+  }
+};
+
+export default {
+  create,
+  schema
+};

tests/rules/assertions/requireCompoundTypeAlias.js

@@ -0,0 +1,92 @@
+export default {
+  invalid: [
+    {
+      code: 'function foo(bar: "A" | "B") {}',
+      errors: [{message: 'All union types must be declared with named type alias.'}]
+    },
+    {
+      code: 'const foo: "A" | "B" = "A";',
+      errors: [{message: 'All union types must be declared with named type alias.'}]
+    },
+    {
+      code: 'type Foo = { bar: "A" | "B" };',
+      errors: [{message: 'All union types must be declared with named type alias.'}]
+    },
+    {
+      code: 'function foo(bar: { n: number } | { s: string }) {}',
+      errors: [{message: 'All union types must be declared with named type alias.'}]
+    },
+    {
+      code: 'function foo(bar: { n: number } & { s: string }) {}',
+      errors: [{message: 'All intersection types must be declared with named type alias.'}]
+    },
+    {
+      code: 'const foo: { n: number } & { s: string } = { n: 0, s: "" };',
+      errors: [{message: 'All intersection types must be declared with named type alias.'}]
+    },
+    {
+      code: 'type Foo = { bar: { n: number } & { s: string } };',
+      errors: [{message: 'All intersection types must be declared with named type alias.'}]
+    },
+    {
+      code: 'function foo(bar: { n: number } & { s: string }) {}',
+      errors: [{message: 'All intersection types must be declared with named type alias.'}]
+    }
+  ],
+  misconfigured: [
+    {
+      errors: [
+        {
+          data: 'sometimes',
+          dataPath: '[0]',
+          keyword: 'enum',
+          message: 'should be equal to one of the allowed values',
+          params: {
+            allowedValues: [
+              'always',
+              'never'
+            ]
+          },
+          parentSchema: {
+            enum: [
+              'always',
+              'never'
+            ],
+            type: 'string'
+          },
+          schema: [
+            'always',
+            'never'
+          ],
+          schemaPath: '#/items/0/enum'
+        }
+      ],
+      options: ['sometimes']
+    }
+  ],
+  valid: [
+    {
+      code: 'type Foo = "A" | "B";'
+    },
+    {
+      code: 'type Bar = "A" | "B"; function foo(bar: Bar) {}'
+    },
+    {
+      code: 'type Foo = { disjoint: "A", n: number } | { disjoint: "B", s: string };'
+    },
+    {
+      code: 'type Foo = { n: number } & { s: string };'
+    },
+    {
+      code: 'type Bar = { n: number } & { s: string }; function foo(bar: Bar) {}'
+    },
+    {
+      code: 'function foo(bar: "A" | "B") {}',
+      options: ['never']
+    },
+    {
+      code: 'function foo(bar: { n: number } & { s: string }) {}',
+      options: ['never']
+    }
+  ]
+};

tests/rules/index.js

@@ -26,6 +26,7 @@ const reportingRules = [
   'no-unused-expressions',
   'no-weak-types',
   'object-type-delimiter',
+  'require-compound-type-alias',
   'require-exact-type',
   'require-parameter-type',
   'require-return-type',