add sparse array check to lint

hello.js

@@ -0,0 +1 @@
+[1,,2]

packages/@romejs/js-compiler/__rtests__/__rsnapshots__/lint.ts.rsnap

@@ -1,4 +1,5 @@
 {
   "empty file": "Object {\n  diagnostics: Array []\n  src: ''\n  ast: Program {\n    body: Array []\n    comments: Array []\n    diagnostics: Array []\n    directives: Array []\n    filename: 'unknown'\n    hasHoistedVars: false\n    interpreter: undefined\n    mtime: undefined\n    sourceType: 'module'\n    syntax: Array []\n    loc: Object {\n      filename: 'unknown'\n      end: Object {\n        column: 0\n        index: 0\n        line: 1\n      }\n      start: Object {\n        column: 0\n        index: 0\n        line: 1\n      }\n    }\n  }\n}",
-  "undeclared variable": "Object {\n  src: 'foobar;'\n  diagnostics: Array [\n    Object {\n      category: 'lint/undeclaredVariables'\n      filename: 'unknown'\n      language: 'js'\n      message: 'Undeclared variable <emphasis>foobar</emphasis>'\n      mtime: undefined\n      sourceType: 'module'\n      origins: Array [Object {category: 'lint'}]\n      end: Object {\n        column: 6\n        index: 6\n        line: 1\n      }\n      start: Object {\n        column: 0\n        index: 0\n        line: 1\n      }\n    }\n  ]\n  ast: Program {\n    comments: Array []\n    diagnostics: Array []\n    directives: Array []\n    filename: 'unknown'\n    hasHoistedVars: false\n    interpreter: undefined\n    mtime: undefined\n    sourceType: 'module'\n    syntax: Array []\n    loc: Object {\n      filename: 'unknown'\n      end: Object {\n        column: 7\n        index: 7\n        line: 1\n      }\n      start: Object {\n        column: 0\n        index: 0\n        line: 1\n      }\n    }\n    body: Array [\n      ExpressionStatement {\n        loc: Object {\n          filename: 'unknown'\n          end: Object {\n            column: 7\n            index: 7\n            line: 1\n          }\n          start: Object {\n            column: 0\n            index: 0\n            line: 1\n          }\n        }\n        expression: ReferenceIdentifier {\n          name: 'foobar'\n          loc: Object {\n            filename: 'unknown'\n            end: Object {\n              column: 6\n              index: 6\n              line: 1\n            }\n            start: Object {\n              column: 0\n              index: 0\n              line: 1\n            }\n          }\n        }\n      }\n    ]\n  }\n}"
+  "undeclared variable": "Object {\n  src: 'foobar;'\n  diagnostics: Array [\n    Object {\n      category: 'lint/undeclaredVariables'\n      filename: 'unknown'\n      language: 'js'\n      message: 'Undeclared variable <emphasis>foobar</emphasis>'\n      mtime: undefined\n      sourceType: 'module'\n      origins: Array [Object {category: 'lint'}]\n      end: Object {\n        column: 6\n        index: 6\n        line: 1\n      }\n      start: Object {\n        column: 0\n        index: 0\n        line: 1\n      }\n    }\n  ]\n  ast: Program {\n    comments: Array []\n    diagnostics: Array []\n    directives: Array []\n    filename: 'unknown'\n    hasHoistedVars: false\n    interpreter: undefined\n    mtime: undefined\n    sourceType: 'module'\n    syntax: Array []\n    loc: Object {\n      filename: 'unknown'\n      end: Object {\n        column: 7\n        index: 7\n        line: 1\n      }\n      start: Object {\n        column: 0\n        index: 0\n        line: 1\n      }\n    }\n    body: Array [\n      ExpressionStatement {\n        loc: Object {\n          filename: 'unknown'\n          end: Object {\n            column: 7\n            index: 7\n            line: 1\n          }\n          start: Object {\n            column: 0\n            index: 0\n            line: 1\n          }\n        }\n        expression: ReferenceIdentifier {\n          name: 'foobar'\n          loc: Object {\n            filename: 'unknown'\n            end: Object {\n              column: 6\n              index: 6\n              line: 1\n            }\n            start: Object {\n              column: 0\n              index: 0\n              line: 1\n            }\n          }\n        }\n      }\n    ]\n  }\n}",
+  "sparse array": "Object {\n  src: '[1,,2]'\n  diagnostics: Array [\n    Object {\n      category: 'lint/sparseArray'\n      filename: 'unknown'\n      language: 'js'\n      message: 'Your array contains an empty slot'\n      mtime: undefined\n      sourceType: 'module'\n      origins: Array [Object {category: 'lint'}]\n      end: Object {\n        column: 6\n        index: 6\n        line: 1\n      }\n      start: Object {\n        column: 0\n        index: 0\n        line: 1\n      }\n    }\n  ]\n  ast: Program {\n    comments: Array []\n    diagnostics: Array []\n    directives: Array []\n    filename: 'unknown'\n    hasHoistedVars: false\n    interpreter: undefined\n    mtime: undefined\n    sourceType: 'module'\n    syntax: Array []\n    loc: Object {\n      filename: 'unknown'\n      end: Object {\n        column: 6\n        index: 6\n        line: 1\n      }\n      start: Object {\n        column: 0\n        index: 0\n        line: 1\n      }\n    }\n    body: Array [\n      ExpressionStatement {\n        loc: Object {\n          filename: 'unknown'\n          end: Object {\n            column: 6\n            index: 6\n            line: 1\n          }\n          start: Object {\n            column: 0\n            index: 0\n            line: 1\n          }\n        }\n        expression: ArrayExpression {\n          loc: undefined\n          elements: Array [\n            NumericLiteral {\n              value: 1\n              loc: Object {\n                filename: 'unknown'\n                end: Object {\n                  column: 2\n                  index: 2\n                  line: 1\n                }\n                start: Object {\n                  column: 1\n                  index: 1\n                  line: 1\n                }\n              }\n            }\n            ReferenceIdentifier {\n              name: 'undefined'\n              loc: undefined\n            }\n            NumericLiteral {\n              value: 2\n              loc: Object {\n                filename: 'unknown'\n                end: Object {\n                  column: 5\n                  index: 5\n                  line: 1\n                }\n                start: Object {\n                  column: 4\n                  index: 4\n                  line: 1\n                }\n              }\n            }\n          ]\n        }\n      }\n    ]\n  }\n}"
 }

packages/@romejs/js-compiler/__rtests__/lint.ts

@@ -79,6 +79,12 @@ test('undeclared variable', async t => {
   ]);
 });
 
+test('sparse array', async t => {
+  const res = await testLint(`[1,,2]`, LINT_ENABLED_FORMAT_DISABLED_CONFIG);
+
+  t.snapshot(res);
+});
+
 test('format disabled in project config should not regenerate the file', async t => {
   // Intentionally weird formatting
   const sourceText = 'foobar ( "yes" );';

packages/@romejs/js-compiler/transforms/lint/index.ts

@@ -9,10 +9,12 @@ import defaultExportSameBasename from './defaultExportSameBasename';
 import undeclaredVariables from './undeclaredVariables';
 import unusedVariables from './unusedVariables';
 import emptyBlocks from './emptyBlocks';
+import sparseArray from './sparseArray';
 
 export const lintTransforms = [
   undeclaredVariables,
   defaultExportSameBasename,
   unusedVariables,
   emptyBlocks,
+  sparseArray,
 ];

packages/@romejs/js-compiler/transforms/lint/sparseArray.ts

@@ -0,0 +1,39 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {Path} from '@romejs/js-compiler';
+import {AnyNode} from '@romejs/js-ast';
+import {
+  referenceIdentifier,
+  arrayExpression,
+  ArrayExpression,
+} from '@romejs/js-ast';
+
+export default {
+  name: 'sparseArray',
+  enter(path: Path): AnyNode {
+    const {node} = path;
+
+    if (node.type === 'ArrayExpression' && node.elements.includes(undefined)) {
+      path.context.addNodeDiagnostic(node, {
+        category: 'lint/sparseArray',
+        message: 'Your array contains an empty slot',
+      });
+      const fixedArray: ArrayExpression = {
+        type: 'ArrayExpression',
+        elements: node.elements.map(elem =>
+          elem === undefined
+            ? referenceIdentifier.create({name: 'undefined'})
+            : elem,
+        ),
+      };
+      return arrayExpression.create(fixedArray);
+    }
+
+    return node;
+  },
+};