test: verify CJS/UMD export contract after every build

lib/cjs.ts instructs verifying the module.exports===Draggable shape with a runtime require() check, but nothing enforced it — the historical CJS dual-export contract (PR #254/#266) and the UMD global could regress invisibly and only surface when a consumer installs the package.

Add scripts/verify-build.cjs: requires the built CJS and asserts root===Draggable, root===.default, and .DraggableCore present; asserts the UMD bundle exposes global ReactDraggable. Wired into the Makefile build target's recipe so it runs after build-lib + build-web on both `yarn build` and `make build`. Verified it fails on a corrupted export shape. Not build-order-coupled to the unit-test matrix.

Author: STRML

Makefile

@@ -19,7 +19,10 @@ lint:
 # tsup emits cjs + esm + dts into build/cjs (and rewrites build/cjs/cjs.js to the
 # legacy module.exports === Draggable shape). webpack emits the UMD global bundle.
 # Both depend on `clean` so the dir is reset first even under parallel make (-j).
+# The recipe runs after both prerequisites complete and verifies the published
+# CJS/UMD contracts (see scripts/verify-build.cjs).
 build: build-lib build-web
+	@node scripts/verify-build.cjs
 
 build-lib: clean $(BIN)
 	$(BIN)/tsup

scripts/verify-build.cjs

@@ -0,0 +1,61 @@
+#!/usr/bin/env node
+'use strict';
+
+// Post-build contract verification.
+//
+// The published artifacts carry two historical public contracts that the build
+// toolchain (tsup for CJS/ESM, webpack for UMD) must preserve. They are easy to
+// break invisibly — a wrong export-interop shape or a missing global only shows
+// up when a consumer installs the package. This script asserts both right after
+// the build, so a regression fails CI instead of shipping. It is wired into the
+// Makefile `build` target (runs after build-lib + build-web).
+
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const path = require('node:path');
+
+const root = path.resolve(__dirname, '..');
+
+// ── Contract 1: CJS export shape (PR #254 / issue #266) ──────────────────────
+// require('react-draggable') returns the Draggable class ITSELF (not a namespace
+// object), and also exposes .default and .DraggableCore. All three access
+// patterns are part of the public API.
+const cjsPath = path.join(root, 'build', 'cjs', 'cjs.js');
+assert.ok(
+  fs.existsSync(cjsPath),
+  `Missing CJS build at ${cjsPath} — run \`yarn build\` first.`
+);
+
+// eslint-disable-next-line global-require, import/no-dynamic-require
+const cjs = require(cjsPath);
+assert.equal(
+  typeof cjs,
+  'function',
+  `CJS root export must be the Draggable component, got ${typeof cjs}`
+);
+assert.equal(
+  cjs.name,
+  'Draggable',
+  `CJS root export should be Draggable, got "${cjs.name}"`
+);
+assert.equal(cjs, cjs.default, 'require("react-draggable") must === .default');
+assert.equal(
+  typeof cjs.DraggableCore,
+  'function',
+  'require("react-draggable").DraggableCore must be exported'
+);
+
+// ── Contract 2: UMD global ───────────────────────────────────────────────────
+// The unpkg bundle exposes the library as global `ReactDraggable`, with react /
+// react-dom consumed as the external globals React / ReactDOM.
+const umdPath = path.join(root, 'build', 'web', 'react-draggable.min.js');
+assert.ok(fs.existsSync(umdPath), `Missing UMD build at ${umdPath}`);
+const umd = fs.readFileSync(umdPath, 'utf8');
+assert.ok(
+  umd.includes('ReactDraggable'),
+  'UMD bundle must expose the global `ReactDraggable`'
+);
+
+console.log(
+  '✓ build contract OK: CJS module.exports===Draggable (+.default, .DraggableCore); UMD global ReactDraggable'
+);