feat: don't require $ReadOnlyArray if variable is initialized with empty array (#337)

* feat: Don't require $ReadOnlyArray if variable is initialized with empty array

* docs: phrasing

Author: pnevyk

Diff for: .README/rules/no-mutable-array.md

@@ -13,4 +13,6 @@ General reasons for using immutable data structures:
 * They always have failure atomicity
 * They are much easier to cache
 
+Note that initialization of a variable with an empty array is considered valid (e.g., `const values: Array<string> = [];`). This behavior resembles the behavior of Flow's [unsealed objects](https://flow.org/en/docs/types/objects/#toc-unsealed-objects), as it is assumed that empty array is intended to be mutated.
+
 <!-- assertions noMutableArray -->

Diff for: src/rules/noMutableArray.js

@@ -1,20 +1,49 @@
+import _ from 'lodash';
+
 const schema = [];
 
+// const x = [];
+const isEmptyArrayLiteral = (node) => {
+  return _.get(node, 'init.type') === 'ArrayExpression' && _.get(node, 'init.elements.length') === 0;
+};
+
+// const x = new Array(); const y = Array();
+const isEmptyArrayInstance = (node) => {
+  if (_.get(node, 'init.type') === 'NewExpression' || _.get(node, 'init.type') === 'CallExpression') {
+    return _.get(node, 'init.callee.name') === 'Array' && _.get(node, 'init.arguments.length') === 0;
+  } else {
+    return false;
+  }
+};
+
+const isAnnotationOfEmptyArrayInit = (node) => {
+  if (_.has(node, 'parent.parent.parent')) {
+    const parent = _.get(node, 'parent.parent.parent');
+    const isVariableDeclaration = _.get(parent, 'type') === 'VariableDeclarator';
+
+    return isVariableDeclaration && (isEmptyArrayLiteral(parent) || isEmptyArrayInstance(parent));
+  } else {
+    return false;
+  }
+};
+
 const create = (context) => {
   return {
     ArrayTypeAnnotation (node) {
-      context.report({
-        fix (fixer) {
-          const rawElementType = context.getSourceCode().getText(node.elementType);
-
-          return fixer.replaceText(node, '$ReadOnlyArray<' + rawElementType + '>');
-        },
-        message: 'Use "$ReadOnlyArray" instead of array shorthand notation',
-        node
-      });
+      if (!isAnnotationOfEmptyArrayInit(node)) {
+        context.report({
+          fix (fixer) {
+            const rawElementType = context.getSourceCode().getText(node.elementType);
+
+            return fixer.replaceText(node, '$ReadOnlyArray<' + rawElementType + '>');
+          },
+          message: 'Use "$ReadOnlyArray" instead of array shorthand notation',
+          node
+        });
+      }
     },
     GenericTypeAnnotation (node) {
-      if (node.id.name === 'Array') {
+      if (node.id.name === 'Array' && !isAnnotationOfEmptyArrayInit(node)) {
         context.report({
           fix (fixer) {
             return fixer.replaceText(node.id, '$ReadOnlyArray');

Diff for: tests/rules/assertions/noMutableArray.js

@@ -9,11 +9,36 @@ export default {
       code: 'type X = string[]',
       errors: [{message: 'Use "$ReadOnlyArray" instead of array shorthand notation'}],
       output: 'type X = $ReadOnlyArray<string>'
+    },
+    {
+      code: 'const values: Array<Array<string>> = [];',
+      errors: [{message: 'Use "$ReadOnlyArray" instead of "Array"'}],
+      output: 'const values: Array<$ReadOnlyArray<string>> = [];'
+    },
+    {
+      code: 'let values: Array<Array<string>>;',
+      errors: [
+        {message: 'Use "$ReadOnlyArray" instead of "Array"'},
+        {message: 'Use "$ReadOnlyArray" instead of "Array"'}
+      ],
+      output: 'let values: $ReadOnlyArray<$ReadOnlyArray<string>>;'
     }
   ],
   valid: [
     {
       code: 'type X = $ReadOnlyArray<string>'
+    },
+    {
+      code: 'const values: Array<$ReadOnlyArray<string>> = [];'
+    },
+    {
+      code: 'const values: $ReadOnlyArray<string>[] = [];'
+    },
+    {
+      code: 'const values: Array<$ReadOnlyArray<string>> = new Array();'
+    },
+    {
+      code: 'const values: Array<$ReadOnlyArray<string>> = Array();'
     }
   ]
 };