diff --git a/textproto.ts b/textproto.ts
index 4c4b92627b..342d74b33e 100644
--- a/textproto.ts
+++ b/textproto.ts
@@ -137,10 +137,13 @@ export class TextProtoReader {
   }
 }
 
-function append(a: Uint8Array, b: Uint8Array): Uint8Array {
+export function append(a: Uint8Array, b: Uint8Array): Uint8Array {
   if (a == null) {
     return b;
   } else {
-    throw Error("Not implemented");
+    const output = new Uint8Array(a.length + b.length);
+    output.set(a, 0);
+    output.set(b, a.length);
+    return output;
   }
 }
diff --git a/textproto_test.ts b/textproto_test.ts
index 57ee99459f..25c12b0e87 100644
--- a/textproto_test.ts
+++ b/textproto_test.ts
@@ -4,7 +4,7 @@
 // license that can be found in the LICENSE file.
 
 import { BufReader } from "./bufio.ts";
-import { TextProtoReader } from "./textproto.ts";
+import { TextProtoReader, append } from "./textproto.ts";
 import { stringsReader } from "./util.ts";
 import {
   test,
@@ -82,3 +82,12 @@ test(async function textprotoReadMIMEHeaderNonCompliant() {
 	}
   */
 });
+
+test(async function textprotoAppend() {
+  const enc = new TextEncoder();
+  const dec = new TextDecoder();
+  const u1 = enc.encode("Hello ");
+  const u2 = enc.encode("World");
+  const joined = append(u1, u2);
+  assertEqual(dec.decode(joined), "Hello World");
+});