diff --git a/esbuild-bug.js b/esbuild-bug.js
new file mode 100644
index 0000000..ea968d7
--- /dev/null
+++ b/esbuild-bug.js
@@ -0,0 +1,56 @@
+// I created https://github.com/evanw/esbuild/issues/1551 for this problem
+
+const esbuild = require('esbuild');
+const fs = require('fs');
+const os = require('os');
+
+function print(caption, code) {
+  console.log(caption, '\n```js\n', code.toString().trim(), '\n```\n');
+}
+
+const esm = `// as ESM
+function unusedFn(a) {
+    return a + 1;
+}
+
+const a = 5;
+let b = 7;
+
+export const c = 2;
+`;
+
+const iife = `// as IIFE
+function unusedFn(a) {
+    return a + 1;
+}
+
+const a = 5;
+let b = 7;
+
+window.c = 2;
+`;
+
+const options = {
+  target: 'es2020',
+  format: 'esm',
+};
+
+let result = esbuild.transformSync(iife, Object.assign({}, options, { format: 'iife' }));
+print('Unused code is removed with format:iife', result.code);
+
+result = esbuild.transformSync(esm, options);
+print('Unused code is kept with format:esm', result.code);
+
+const tmpFile = `${os.tmpdir()}/esbuild-test-${Date.now()}.js`;
+fs.writeFileSync(tmpFile, esm, { encoding: 'utf8' });
+esbuild.buildSync(
+  Object.assign(options, {
+    entryPoints: [tmpFile],
+    outfile: tmpFile,
+    allowOverwrite: true,
+    bundle: true,
+  })
+);
+result = fs.readFileSync(tmpFile, { encoding: 'utf8' });
+fs.rmSync(tmpFile);
+print('Unused code is removed with format:esm and bundle:true', result);