js_parser: set p.delete_target before visiting the delete operand

src/js_parser/visit/visit_expr.rs

@@ -1057,7 +1057,14 @@
         let target = e_.target.unwrap_inlined();
         let index = e_.index.unwrap_inlined();
 
-        if p.options.features.minify_syntax {
+        // `a[n]` is a property reference; replacing it with a value changes
+        // the result of `delete` (reference → value makes it return `true`)
+        // and of assignment (writes to the wrong place). Neither `"foo"[2]`
+        // → `"o"` nor `[x][0]` → `x` is safe in those positions.
+        if p.options.features.minify_syntax
+            && !is_delete_target
+            && in_.assign_target == js_ast::AssignTarget::None
+        {
             if let Some(number) = index.data.as_e_number() {
                 if number.value() >= 0.0
                     && number.value() < (usize::MAX as f64)
@@ -1218,6 +1225,7 @@
                 }
             }
             Op::UnDelete => {
+                p.delete_target = e_.value.data;
                 p.visit_expr_in_out(&mut e_.value, ExprIn::default());
             }
             _ => {

src/jsc/RuntimeTranspilerCache.rs

@@ -48,7 +48,10 @@ bun_core::declare_scope!(cache, visible);
 /// bindings from the compiled bytecode after the module-loader rewrite, so the
 /// record no longer carries them; blobs written in the old numbering must not
 /// be read back.
-const EXPECTED_VERSION: u32 = 24;
+/// Version 25: `p.delete_target` is now set before visiting the `delete`
+/// operand, so `delete E.Member` / `delete {f:x}.f` / `delete [x][0]` /
+/// `delete "s"[n]` no longer fold to a value under `minify_syntax`.
+const EXPECTED_VERSION: u32 = 25;
 
 /// Source files smaller than this are not written to / read from the on-disk
 /// transpiler cache. Originally 50 KiB, which excluded almost every file in a

test/bundler/bundler_cjs2esm.test.ts

@@ -392,6 +392,36 @@ describe("bundler", () => {
       stdout: '[[{"xyz":456},456],[{"xyz":123},123],[{"xyz":456},456],[{"xyz":123},123]]',
     },
   });
+  itBundled("cjs2esm/DeleteExportsPropertyDeopt", {
+    files: {
+      "/entry.js": /* js */ `
+        import * as lib from './lib.js';
+        console.log(lib.a, lib.b);
+      `,
+      "/lib.js": /* js */ `
+        exports.a = 1;
+        exports.b = 2;
+        delete exports.a;
+      `,
+    },
+    cjs2esm: { unhandled: ["/lib.js"] },
+    run: { stdout: "undefined 2" },
+  });
+  itBundled("cjs2esm/DeleteModuleExportsPropertyDeopt", {
+    files: {
+      "/entry.js": /* js */ `
+        import * as lib from './lib.js';
+        console.log(lib.a, lib.b);
+      `,
+      "/lib.js": /* js */ `
+        module.exports.a = 1;
+        module.exports.b = 2;
+        delete module.exports.a;
+      `,
+    },
+    cjs2esm: { unhandled: ["/lib.js"] },
+    run: { stdout: "undefined 2" },
+  });
   // https://github.com/oven-sh/bun/issues/4565
   // `exports.x = ...` as the unbraced body of if/while/do/else must not be
   // converted to `var $x = ...; export { $x as x };` because `export` is only

test/bundler/transpiler/assign-to-import.test.ts

@@ -81,6 +81,17 @@ describe("assigning to an imported binding", () => {
       `import * as ns from "./m.mjs"; const k = "x"; ns[k] = 5;\n`,
       'Cannot assign to property on import "ns"',
     ],
+    ["delete namespace property", `import * as ns from "./m.mjs"; delete ns.x;\n`, 'Cannot assign to import "x"'],
+    [
+      "delete string-index namespace property",
+      `import * as ns from "./m.mjs"; delete ns["x"];\n`,
+      'Cannot assign to import "x"',
+    ],
+    [
+      "delete computed namespace property",
+      `import * as ns from "./m.mjs"; const k = "x"; delete ns[k];\n`,
+      'Cannot assign to property on import "ns"',
+    ],
   ])("bun build still rejects it: %s", async (_name, entry, diagnostic) => {
     using dir = tempDir("assign-to-import-build", {
       "m.mjs": mod,

test/bundler/transpiler/transpiler.test.js

@@ -139,6 +139,57 @@ describe("Bun.Transpiler", () => {
     it("works nested", () => {
       ts.expectPrintedMin_('const a = ["hey"][0][0];', 'const a = "h"');
     });
+    it("bails out when the index is a delete or assignment target", () => {
+      // `a[n]` is a property reference; folding it to a value changes the
+      // result of `delete` and the effect of assignment.
+      ts.expectPrintedMin_("x = delete [y][0]", "x = delete [y][0]");
+      ts.expectPrintedMin_("x = delete [y.z][0]", "x = delete [y.z][0]");
+      ts.expectPrintedMin_("x = delete { f: y }.f", "x = delete { f: y }.f");
+      ts.expectPrintedMin_("x = delete { f: y }['f']", "x = delete { f: y }.f");
+      ts.expectPrintedMin_('x = delete "foo"[2]', 'x = delete "foo"[2]');
+      ts.expectPrintedMin_("x = [y][0] = 5", "x = [y][0] = 5");
+      ts.expectPrintedMin_("x = [y][0] += 5", "x = [y][0] += 5");
+      ts.expectPrintedMin_("[y][0]++", "[y][0]++");
+      ts.expectPrintedMin_('x = "foo"[2] = 5', 'x = "foo"[2] = 5');
+      // Still inlined outside those positions.
+      ts.expectPrintedMin_("x = [y][0]", "x = y");
+      ts.expectPrintedMin_('x = "foo"[2]', 'x = "o"');
+      ts.expectPrintedMin_("x = delete [y][0].z", "x = delete y.z");
+    });
+    it("does not inline an enum member under delete", () => {
+      const pre = "enum E { A = 1 }\n";
+      const lastLine = out => out.trimEnd().split("\n").at(-1);
+      expect(lastLine(ts.parsed(pre + "x = delete E.A;", false))).toBe("x = delete E.A;");
+      expect(lastLine(ts.parsed(pre + 'x = delete E["A"];', false))).toBe('x = delete E["A"];');
+      expect(lastLine(ts.parsedMin(pre + "x = delete E.A;", false))).toBe("x = delete E.A;");
+      // Still inlined when read.
+      expect(lastLine(ts.parsed(pre + "x = E.A;", false))).toBe("x = 1 /* A */;");
+    });
+    it("does not substitute --define for a delete target", () => {
+      // `user_undefined` is defined as `undefined` in the transpiler config above.
+      ts.expectPrintedMin_("x = delete user_undefined", "x = delete user_undefined");
+      // A later read is still substituted (delete_target is per-node, not per-symbol).
+      ts.expectPrintedMin_(
+        "x = delete user_undefined; y = user_undefined;",
+        "x = delete user_undefined;\ny = void 0;\n",
+      );
+    });
+    it("preserves delete/assign semantics at runtime", async () => {
+      const src = `
+        var obj = { p: 1 };
+        var r1 = delete [obj.p][0];
+        var y = 1;
+        [y][0] = 5;
+        var k = { f: 7 };
+        var r2 = delete { f: k.f }.f;
+        console.log(JSON.stringify([obj.p, r1, y, k.f, r2]));
+      `;
+      await using proc = Bun.spawn({ cmd: [bunExe(), "-e", src], env: bunEnv, stderr: "pipe" });
+      const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+      expect(stderr).toBe("");
+      expect(stdout).toBe("[1,true,1,7,true]\n");
+      expect(exitCode).toBe(0);
+    });
     it("bails out on optional-chain index into enum", () => {
       const pre = "enum Foo { A }\nenum Bar { 'a-b' = 1 }\n";
       const lastLine = out => out.trimEnd().split("\n").at(-1);