polyfill .flat() array method

Author: lobsterkatie

packages/utils/package.json

@@ -20,6 +20,8 @@
     "tslib": "^1.9.3"
   },
   "devDependencies": {
+    "@types/array.prototype.flat": "^1.2.1",
+    "array.prototype.flat": "^1.3.0",
     "chai": "^4.1.2"
   },
   "scripts": {

packages/utils/test/buildPolyfills/optionalChain.test.ts

@@ -1,7 +1,13 @@
+import { default as arrayFlat } from 'array.prototype.flat';
+
 import { _optionalChain } from '../../src/buildPolyfills';
 import { GenericFunction, GenericObject, Value } from '../../src/buildPolyfills/types';
 import { _optionalChain as _optionalChainOrig } from './originals';
 
+// Older versions of Node don't have `Array.prototype.flat`, which crashes these tests. On newer versions that do have
+// it, this is a no-op.
+arrayFlat.shim();
+
 type OperationType = 'access' | 'call' | 'optionalAccess' | 'optionalCall';
 type OperationExecutor =
   | ((intermediateValue: GenericObject) => Value)

packages/utils/tsconfig.test.json

@@ -9,6 +9,15 @@
 
     // other package-specific, test-specific options
     // this is necessary in order to be able to handle the buildPolyfills `originals.js` which is used for testing
-    "allowJs": true
+    "allowJs": true,
+
+    // `es2020` is the recommended `lib` and `target` for Node 14
+    // see https://github.com/tsconfig/bases/blob/main/bases/node14.json
+    "lib": ["dom", "es2020"],
+    "module": "commonjs",
+    "target": "es2020",
+
+    // so we don't have to worry about how libraries we use export things
+    "esModuleInterop": true
   }
 }