fix #4286: minifier panic due to identity function

evanw · evanw · commit 73a0b2ae491c · 2025-09-15T23:05:46.000-04:00
diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -2,6 +2,15 @@
 
 ## Unreleased
 
+* Fix a panic in a minification edge case ([#4286](https://github.com/evanw/esbuild/issues/4286))
+
+    This release fixes a panic due to a null pointer that could happen when esbuild inlines a doubly-nested identity function and the final result is empty. It was fixed by emitting the value `undefined` in this case, which avoids the panic. This case must be rare since it hasn't come up until now. Here is an example of code that previously triggered the panic (which only happened when minifying):
+
+    ```js
+    function identity(x) { return x }
+    identity({ y: identity(123) })
+    ```
+
 * Fix `@supports` nested inside pseudo-element ([#4265](https://github.com/evanw/esbuild/issues/4265))
 
     When transforming nested CSS to non-nested CSS, esbuild is supposed to filter out pseudo-elements such as `::placeholder` for correctness. The [CSS nesting specification](https://www.w3.org/TR/css-nesting-1/) says the following:
diff --git a/internal/bundler_tests/bundler_dce_test.go b/internal/bundler_tests/bundler_dce_test.go
@@ -2594,6 +2594,12 @@ func TestInlineIdentityFunctionCalls(t *testing.T) {
 				keep(foo())
 				keep(1)
 			`,
+
+			"/identity-simplify-unused-issue-4287.js": `
+				function id(x) { return x }
+				id({ x: id([123, foo()]) })
+				id({ x: id(123) })
+			`,
 		},
 		entryPaths: []string{
 			"/identity.js",
@@ -2615,6 +2621,7 @@ func TestInlineIdentityFunctionCalls(t *testing.T) {
 			"/not-identity-object.js",
 			"/not-identity-rest.js",
 			"/not-identity-return.js",
+			"/identity-simplify-unused-issue-4287.js",
 		},
 		options: config.Options{
 			Mode:         config.ModeBundle,
diff --git a/internal/bundler_tests/snapshots/snapshots_dce.txt b/internal/bundler_tests/snapshots/snapshots_dce.txt
@@ -1483,6 +1483,11 @@ console.log(keep(1));
 keep(foo());
 keep(1);
 
+---------- /out/identity-simplify-unused-issue-4287.js ----------
+// identity-simplify-unused-issue-4287.js
+foo();
+void 0;
+
 ================================================================================
 TestJSONLoaderRemoveUnused
 ---------- /out.js ----------
diff --git a/internal/js_printer/js_printer.go b/internal/js_printer/js_printer.go
@@ -2333,6 +2333,9 @@ func (p *printer) printExpr(expr js_ast.Expr, level js_ast.L, flags printExprFla
 				if _, ok := arg.Data.(*js_ast.ESpread); !ok {
 					if (flags & exprResultIsUnused) != 0 {
 						arg = p.astHelpers.SimplifyUnusedExpr(arg, p.options.UnsupportedFeatures)
+						if arg.Data == nil {
+							arg.Data = js_ast.EUndefinedShared
+						}
 					}
 					p.printExpr(p.guardAgainstBehaviorChangeDueToSubstitution(arg, flags), level, flags)
 					break
