fix(wasi): add missing initialize() and getImportObject() methods

The node:wasi WASI class was missing the `initialize()` method needed
for WASI reactor modules, and the `getImportObject()` convenience method.
This caused a TypeError when libraries like @ruby/wasm-wasi tried to call
`wasi.initialize()`.

Closes #12755

Co-Authored-By: Claude <noreply@anthropic.com>

src/js/node/wasi.ts

@@ -1863,6 +1863,25 @@ var require_wasi = __commonJS({
           exports2._start();
         }
       }
+      initialize(instance, memory) {
+        const exports2 = instance.exports;
+        if (exports2 === null || typeof exports2 !== "object") {
+          throw new Error(`instance.exports must be an Object. Received ${exports2}.`);
+        }
+        if (memory == null) {
+          memory = exports2.memory;
+          if (!(memory instanceof WebAssembly.Memory)) {
+            throw new Error(`instance.exports.memory must be a WebAssembly.Memory. Received ${memory}.`);
+          }
+        }
+        this.setMemory(memory);
+        if (typeof exports2._start === "function") {
+          throw new Error("instance.exports._start must not exist for WASI reactors. Use start() for WASI commands.");
+        }
+        if (typeof exports2._initialize === "function") {
+          exports2._initialize();
+        }
+      }
       getImports(module2) {
         let namespace: string | null = null;
         const imports = WebAssembly.Module.imports(module2);
@@ -1897,6 +1916,9 @@ var require_wasi = __commonJS({
           }
         }
       }
+      getImportObject() {
+        return { wasi_snapshot_preview1: this.wasiImport };
+      }
       initWasiFdInfo() {
         if (this.env["WASI_FD_INFO"] != null) {
           const fdInfo = JSON.parse(this.env["WASI_FD_INFO"]);

test/regression/issue/12755.test.ts

@@ -0,0 +1,141 @@
+import { describe, expect, test } from "bun:test";
+import { bunEnv, bunExe } from "harness";
+
+describe("WASI", () => {
+  test("initialize() method exists on WASI instance", async () => {
+    await using proc = Bun.spawn({
+      cmd: [
+        bunExe(),
+        "-e",
+        `
+const { WASI } = require("node:wasi");
+const wasi = new WASI({ version: "preview1" });
+console.log(typeof wasi.initialize);
+`,
+      ],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    expect(stdout.trim()).toBe("function");
+    expect(exitCode).toBe(0);
+  });
+
+  test("getImportObject() method exists on WASI instance", async () => {
+    await using proc = Bun.spawn({
+      cmd: [
+        bunExe(),
+        "-e",
+        `
+const { WASI } = require("node:wasi");
+const wasi = new WASI({ version: "preview1" });
+console.log(typeof wasi.getImportObject);
+`,
+      ],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    expect(stdout.trim()).toBe("function");
+    expect(exitCode).toBe(0);
+  });
+
+  test("initialize() works with a reactor WASI module (no _start, with _initialize)", async () => {
+    await using proc = Bun.spawn({
+      cmd: [
+        bunExe(),
+        "-e",
+        `
+const { WASI } = require("node:wasi");
+const wasi = new WASI({ version: "preview1" });
+
+// Minimal WASI reactor module: exports memory, _initialize (nop), add (i32.add)
+const bytes = new Uint8Array([0,97,115,109,1,0,0,0,1,10,2,96,0,0,96,2,127,127,1,127,3,3,2,0,1,5,3,1,0,1,7,30,3,6,109,101,109,111,114,121,2,0,11,95,105,110,105,116,105,97,108,105,122,101,0,0,3,97,100,100,0,1,10,12,2,2,0,11,7,0,32,0,32,1,106,11]);
+
+const module = new WebAssembly.Module(bytes);
+const instance = new WebAssembly.Instance(module);
+
+// This should NOT throw - it's the core fix for #12755
+wasi.initialize(instance);
+
+// Verify the module still works after initialization
+const result = instance.exports.add(2, 3);
+console.log(result);
+`,
+      ],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    expect(stdout.trim()).toBe("5");
+    expect(exitCode).toBe(0);
+  });
+
+  test("initialize() throws if _start is exported", async () => {
+    await using proc = Bun.spawn({
+      cmd: [
+        bunExe(),
+        "-e",
+        `
+const { WASI } = require("node:wasi");
+const wasi = new WASI({ version: "preview1" });
+
+// WASI command module: exports memory and _start (nop)
+const bytes = new Uint8Array([0,97,115,109,1,0,0,0,1,4,1,96,0,0,3,2,1,0,5,3,1,0,1,7,19,2,6,109,101,109,111,114,121,2,0,6,95,115,116,97,114,116,0,0,10,4,1,2,0,11]);
+
+const module = new WebAssembly.Module(bytes);
+const instance = new WebAssembly.Instance(module);
+
+try {
+  wasi.initialize(instance);
+  console.log("ERROR: should have thrown");
+  process.exit(1);
+} catch (e) {
+  console.log("caught: " + e.message.includes("_start"));
+}
+`,
+      ],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    expect(stdout.trim()).toBe("caught: true");
+    expect(exitCode).toBe(0);
+  });
+
+  test("getImportObject() returns object with wasi_snapshot_preview1 key", async () => {
+    await using proc = Bun.spawn({
+      cmd: [
+        bunExe(),
+        "-e",
+        `
+const { WASI } = require("node:wasi");
+const wasi = new WASI({ version: "preview1" });
+const importObj = wasi.getImportObject();
+console.log(typeof importObj.wasi_snapshot_preview1);
+console.log(typeof importObj.wasi_snapshot_preview1.fd_write);
+`,
+      ],
+      env: bunEnv,
+      stdout: "pipe",
+      stderr: "pipe",
+    });
+
+    const [stdout, stderr, exitCode] = await Promise.all([proc.stdout.text(), proc.stderr.text(), proc.exited]);
+
+    expect(stdout.trim()).toBe("object\nfunction");
+    expect(exitCode).toBe(0);
+  });
+});