NEW RULE: avoid-duplicate-fields

.changeset/clean-apricots-attend.md

@@ -0,0 +1,5 @@
+---
+'@graphql-eslint/eslint-plugin': minor
+---
+
+NEW RULE: avoid-duplicate-fields

docs/README.md

@@ -15,9 +15,10 @@
 - [`require-description`](./rules/require-description.md)
 - [`require-id-when-available`](./rules/require-id-when-available.md)
 - [`description-style`](./rules/description-style.md)
-- [`prettier`](./rules/prettier.md)
+- [`avoid-duplicate-fields`](./rules/avoid-duplicate-fields.md)
 - [`naming-convention`](./rules/naming-convention.md)
 - [`input-name`](./rules/input-name.md)
+- [`prettier`](./rules/prettier.md)
 - [`executable-definitions`](./rules/executable-definitions.md)
 - [`fields-on-correct-type`](./rules/fields-on-correct-type.md)
 - [`fragments-on-composite-type`](./rules/fragments-on-composite-type.md)

docs/rules/avoid-duplicate-fields.md

@@ -0,0 +1,54 @@
+# `avoid-duplicate-fields`
+
+- Category: `Stylistic Issues`
+- Rule name: `@graphql-eslint/avoid-duplicate-fields`
+- Requires GraphQL Schema: `false` [ℹ️](../../README.md#extended-linting-rules-with-graphql-schema)
+- Requires GraphQL Operations: `false` [ℹ️](../../README.md#extended-linting-rules-with-siblings-operations)
+
+Checks for duplicate fields in selection set, variables in operation definition, or in arguments set of a field.
+
+## Usage Examples
+
+### Incorrect
+
+```graphql
+# eslint @graphql-eslint/avoid-duplicate-fields: ["error"]
+
+query getUserDetails {
+  user {
+    name # first
+    email
+    name # second
+  }
+}
+```
+
+### Incorrect
+
+```graphql
+# eslint @graphql-eslint/avoid-duplicate-fields: ["error"]
+
+query getUsers {
+  users(
+    first: 100
+    skip: 50
+    after: "cji629tngfgou0b73kt7vi5jo"
+    first: 100 # duplicate argument
+  ) {
+    id
+  }
+}
+```
+
+### Incorrect
+
+```graphql
+# eslint @graphql-eslint/avoid-duplicate-fields: ["error"]
+
+query getUsers($first: Int!, $first: Int!) {
+  # Duplicate variable
+  users(first: 100, skip: 50, after: "cji629tngfgou0b73kt7vi5jo") {
+    id
+  }
+}
+```

packages/plugin/src/rules/avoid-duplicate-fields.ts

@@ -0,0 +1,133 @@
+import { Kind } from 'graphql';
+import { GraphQLESLintRule } from '../types';
+
+const AVOID_DUPLICATE_FIELDS = 'AVOID_DUPLICATE_FIELDS';
+
+const ensureUnique = () => {
+  const set = new Set<string>();
+
+  return {
+    add: (item: string, onError: () => void) => {
+      if (set.has(item)) {
+        onError();
+      } else {
+        set.add(item);
+      }
+    },
+  };
+};
+
+const rule: GraphQLESLintRule<[], false> = {
+  meta: {
+    type: 'suggestion',
+    docs: {
+      requiresSchema: false,
+      requiresSiblings: false,
+      description:
+        'Checks for duplicate fields in selection set, variables in operation definition, or in arguments set of a field.',
+      category: 'Stylistic Issues',
+      url: 'https://github.com/dotansimha/graphql-eslint/blob/master/docs/rules/avoid-duplicate-fields.md',
+      examples: [
+        {
+          title: 'Incorrect',
+          code: /* GraphQL */ `
+            query getUserDetails {
+              user {
+                name # first
+                email
+                name # second
+              }
+            }
+          `,
+        },
+        {
+          title: 'Incorrect',
+          code: /* GraphQL */ `
+            query getUsers {
+              users(
+                first: 100
+                skip: 50
+                after: "cji629tngfgou0b73kt7vi5jo"
+                first: 100 # duplicate argument
+              ) {
+                id
+              }
+            }
+          `,
+        },
+        {
+          title: 'Incorrect',
+          code: /* GraphQL */ `
+            query getUsers($first: Int!, $first: Int!) {
+              # Duplicate variable
+              users(first: 100, skip: 50, after: "cji629tngfgou0b73kt7vi5jo") {
+                id
+              }
+            }
+          `,
+        },
+      ],
+    },
+    messages: {
+      [AVOID_DUPLICATE_FIELDS]: `{{ type }} "{{ fieldName }}" defined multiple times.`,
+    },
+  },
+  create(context) {
+    return {
+      OperationDefinition(node) {
+        const uniqueCheck = ensureUnique();
+
+        for (const arg of node.variableDefinitions || []) {
+          uniqueCheck.add(arg.variable.name.value, () => {
+            context.report({
+              messageId: AVOID_DUPLICATE_FIELDS,
+              data: {
+                type: 'Operation variable',
+                fieldName: arg.variable.name.value,
+              },
+              node: arg,
+            });
+          });
+        }
+      },
+      Field(node) {
+        const uniqueCheck = ensureUnique();
+
+        for (const arg of node.arguments || []) {
+          uniqueCheck.add(arg.name.value, () => {
+            context.report({
+              messageId: AVOID_DUPLICATE_FIELDS,
+              data: {
+                type: 'Field argument',
+                fieldName: arg.name.value,
+              },
+              node: arg,
+            });
+          });
+        }
+      },
+      SelectionSet(node) {
+        const uniqueCheck = ensureUnique();
+
+        for (const selection of node.selections || []) {
+          if (selection.kind === Kind.FIELD) {
+            const nameToCheck = selection.alias?.value || selection.name.value;
+
+            uniqueCheck.add(nameToCheck, () => {
+              context.report({
+                messageId: AVOID_DUPLICATE_FIELDS,
+                data: {
+                  type: 'Field',
+                  fieldName: nameToCheck,
+                },
+                node: selection,
+              });
+            });
+          }
+        }
+      },
+    };
+  },
+};
+
+export default rule;

packages/plugin/src/rules/index.ts

@@ -15,6 +15,7 @@ import uniqueOperationName from './unique-operation-name';
 import noDeprecated from './no-deprecated';
 import noHashtagDescription from './no-hashtag-description';
 import selectionSetDepth from './selection-set-depth';
+import avoidDuplicateFields from './avoid-duplicate-fields';
 import { GraphQLESLintRule } from '../types';
 import { GRAPHQL_JS_VALIDATIONS } from './graphql-js-validation';
 
@@ -33,8 +34,9 @@ export const rules: Record<string, GraphQLESLintRule> = {
   'require-description': requireDescription,
   'require-id-when-available': requireIdWhenAvailable,
   'description-style': descriptionStyle,
-  prettier: prettier,
+  'avoid-duplicate-fields': avoidDuplicateFields,
   'naming-convention': namingConvention,
   'input-name': inputName,
+  prettier,
   ...GRAPHQL_JS_VALIDATIONS,
 };

packages/plugin/tests/avoid-duplicate-fields.spec.ts

@@ -0,0 +1,54 @@
+import { GraphQLRuleTester } from '../src/testkit';
+import rule from '../src/rules/avoid-duplicate-fields';
+
+const ruleTester = new GraphQLRuleTester();
+
+ruleTester.runGraphQLTests('avoid-duplicate-fields', rule, {
+  valid: [],
+  invalid: [
+    {
+      code: /* GraphQL */ `
+        query test($v: String, $t: String, $v: String) {
+          id
+        }
+      `,
+      errors: [{ message: 'Operation variable "v" defined multiple times.' }],
+    },
+    {
+      code: /* GraphQL */ `
+        query test {
+          users(first: 100, after: 10, filter: "test", first: 50) {
+            id
+          }
+        }
+      `,
+      errors: [{ message: 'Field argument "first" defined multiple times.' }],
+    },
+    {
+      code: /* GraphQL */ `
+        query test {
+          users {
+            id
+            name
+            email
+            name
+          }
+        }
+      `,
+      errors: [{ message: 'Field "name" defined multiple times.' }],
+    },
+    {
+      code: /* GraphQL */ `
+        query test {
+          users {
+            id
+            name
+            email
+            email: somethingElse
+          }
+        }
+      `,
+      errors: [{ message: 'Field "email" defined multiple times.' }],
+    },
+  ],
+});