diff --git a/core/src/main/java/io/roastedroot/quickjs4j/core/BasicScriptCache.java b/core/src/main/java/io/roastedroot/quickjs4j/core/BasicScriptCache.java
index 9af0196..30b91da 100644
--- a/core/src/main/java/io/roastedroot/quickjs4j/core/BasicScriptCache.java
+++ b/core/src/main/java/io/roastedroot/quickjs4j/core/BasicScriptCache.java
@@ -1,6 +1,6 @@
 package io.roastedroot.quickjs4j.core;
 
-import java.nio.charset.StandardCharsets;
+import java.nio.ByteBuffer;
 import java.security.MessageDigest;
 import java.security.NoSuchAlgorithmException;
 import java.util.HashMap;
@@ -8,7 +8,7 @@
 public class BasicScriptCache implements ScriptCache, AutoCloseable {
     private static final String DEFAULT_MESSAGE_DIGEST_ALGORITHM = "SHA-256";
 
-    private final HashMap<String, byte[]> cache;
+    private final HashMap<ByteBuffer, byte[]> cache;
     private final MessageDigest messageDigest;
 
     public BasicScriptCache() {
@@ -24,19 +24,20 @@ public BasicScriptCache(String messageDigestAlgorithm) {
         }
     }
 
+    private ByteBuffer key(byte[] code) {
+        return ByteBuffer.wrap(messageDigest.digest(code));
+    }
+
     public boolean exists(byte[] code) {
-        var key = messageDigest.digest(code);
-        return cache.containsKey(new String(key, StandardCharsets.UTF_8));
+        return cache.containsKey(key(code));
     }
 
     public void set(byte[] code, byte[] compiled) {
-        var key = messageDigest.digest(code);
-        cache.put(new String(key, StandardCharsets.UTF_8), compiled);
+        cache.put(key(code), compiled);
     }
 
     public byte[] get(byte[] code) {
-        var key = messageDigest.digest(code);
-        return cache.get(new String(key, StandardCharsets.UTF_8));
+        return cache.get(key(code));
     }
 
     public void close() {
diff --git a/core/src/main/java/io/roastedroot/quickjs4j/core/Engine.java b/core/src/main/java/io/roastedroot/quickjs4j/core/Engine.java
index e91ec1d..c02f606 100644
--- a/core/src/main/java/io/roastedroot/quickjs4j/core/Engine.java
+++ b/core/src/main/java/io/roastedroot/quickjs4j/core/Engine.java
@@ -29,6 +29,8 @@
 @WasmModuleInterface(WasmResource.absoluteFile)
 public final class Engine implements AutoCloseable {
     private static final int ALIGNMENT = 1;
+
+    private static final byte[] NULL_BYTES = "null".getBytes(UTF_8);
     public static final ObjectMapper DEFAULT_OBJECT_MAPPER = new ObjectMapper();
 
     private final ByteArrayOutputStream stdout;
@@ -120,11 +122,16 @@ private String readJavyString(int ptr, int len) {
 
     public Object invokeGuestFunction(
             String moduleName, String name, List<Object> args, String libraryCode) {
+        return invokeGuestFunction(moduleName, name, args, libraryCode.getBytes(UTF_8));
+    }
+
+    public Object invokeGuestFunction(
+            String moduleName, String name, List<Object> args, byte[] libraryCode) {
         return invokePrecompiledGuestFunction(
                 moduleName, name, args, compilePortableGuestFunction(libraryCode));
     }
 
-    public String invokeFunction() {
+    private String invokeFunction() {
         var funInvoke =
                 "globalThis[quickjs4j_engine.module_name()][quickjs4j_engine.function_name()](...JSON.parse(quickjs4j_engine.args()))";
         Function<String, String> setResult =
@@ -146,24 +153,24 @@ public String invokeFunction() {
     // we compile a static version of the module and we invoke it parametrically using a basic
     // protocol
     // { moduleName: "..", functionName: "..", args: "stringified args" }
+    public byte[] compilePortableGuestFunction(byte[] libraryCode) {
+        return compilePortableGuestFunction(new String(libraryCode, UTF_8));
+    }
+
     public byte[] compilePortableGuestFunction(String libraryCode) {
         int codePtr = 0;
         try {
-            var invokeFunction = invokeFunction();
-
-            String jsCode =
-                    new String(jsPrelude(), UTF_8)
-                            + "\n"
-                            + libraryCode
-                            + "\n"
-                            + new String(jsSuffix(), UTF_8)
-                            + "\n"
-                            + invokeFunction
-                            + ";\n";
-
-            // System.out.println(jsCode);
-
-            codePtr = compileRaw(jsCode.getBytes(UTF_8));
+            var buf = new StringBuilder();
+            buf.append(jsPrelude());
+            buf.append('\n');
+            buf.append(libraryCode);
+            buf.append('\n');
+            buf.append(jsSuffix());
+            buf.append('\n');
+            buf.append(invokeFunction());
+            buf.append(";\n");
+
+            codePtr = compileRaw(buf.toString().getBytes(UTF_8));
             return readCompiled(codePtr);
         } finally {
             if (codePtr != 0) {
@@ -267,11 +274,10 @@ private long[] invokeBuiltin(Instance instance, long[] args) {
                 }
             }
 
-            var returnStr =
+            var returnBytes =
                     (returnType == Void.class)
-                            ? "null"
-                            : mapper.writerFor(returnType).writeValueAsString(res);
-            var returnBytes = returnStr.getBytes();
+                            ? NULL_BYTES
+                            : mapper.writerFor(returnType).writeValueAsBytes(res);
 
             var returnPtr =
                     exports.canonicalAbiRealloc(
@@ -315,7 +321,7 @@ private long[] invokeBuiltin(Instance instance, long[] args) {
                     this::invokeBuiltin);
 
     // This function dynamically generates the global functions defined by the Builtins
-    private byte[] jsPrelude() {
+    private String jsPrelude() {
         var preludeBuilder = new StringBuilder();
         for (Map.Entry<String, Builtins> builtin : builtins.entrySet()) {
             preludeBuilder.append("globalThis." + builtin.getKey() + " = {};\n");
@@ -332,11 +338,11 @@ private byte[] jsPrelude() {
                                 + "\", JSON.stringify(args))) };\n");
             }
         }
-        return preludeBuilder.toString().getBytes();
+        return preludeBuilder.toString();
     }
 
     // This function dynamically generates the js handlers for Invokables
-    private byte[] jsSuffix() {
+    private String jsSuffix() {
         var suffixBuilder = new StringBuilder();
         for (Map.Entry<String, Invokables> invokable : invokables.entrySet()) {
             // The object is already defined by the set_result, just add the handlers
@@ -352,7 +358,7 @@ private byte[] jsSuffix() {
                                 + ";\n");
             }
         }
-        return suffixBuilder.toString().getBytes();
+        return suffixBuilder.toString();
     }
 
     public int compile(String js) {
@@ -360,7 +366,7 @@ public int compile(String js) {
     }
 
     public int compile(byte[] js) {
-        byte[] prelude = jsPrelude();
+        byte[] prelude = jsPrelude().getBytes(UTF_8);
         byte[] jsCode = new byte[prelude.length + js.length];
         System.arraycopy(prelude, 0, jsCode, 0, prelude.length);
         System.arraycopy(js, 0, jsCode, prelude.length, js.length);
diff --git a/core/src/main/java/io/roastedroot/quickjs4j/core/Runner.java b/core/src/main/java/io/roastedroot/quickjs4j/core/Runner.java
index 81f0605..b6b2e79 100644
--- a/core/src/main/java/io/roastedroot/quickjs4j/core/Runner.java
+++ b/core/src/main/java/io/roastedroot/quickjs4j/core/Runner.java
@@ -24,10 +24,13 @@ private Runner(Engine engine, int timeout, int compilationTimeout, ExecutorServi
     }
 
     public byte[] compile(String code) {
+        return compile(code.getBytes(StandardCharsets.UTF_8));
+    }
+
+    public byte[] compile(byte[] code) {
         return submitWithTimeout(
                 () -> {
-                    byte[] codeBytes = code.getBytes(StandardCharsets.UTF_8);
-                    int codePtr = engine.compile(codeBytes);
+                    int codePtr = engine.compile(code);
                     try {
                         return engine.readCompiled(codePtr);
                     } finally {