download button (not tested yet)

index.pug

@@ -30,8 +30,7 @@ html(lang="en-gb")
                 h2 download
                 textarea#output
                 br
-                //-
-                    button#download download
+                button#download download
                 button.pagination.prev#add add another image
                 button.pagination.start#reset start again
         script(type="module")

script.js

@@ -1,5 +1,17 @@
 import init, {load_default_game_data, add_tiles} from './pkg/bitsy_tiles.js';
 
+// stolen from https://ourcodeworld.com/articles/read/189/how-to-create-a-file-and-generate-a-download-with-javascript-in-the-browser-without-a-server
+function download(filename, text) {
+    let element = document.createElement('a');
+
+    element.setAttribute('href', 'data:text/plain;charset=utf-8,' + encodeURIComponent(text));
+    element.setAttribute('download', filename);
+    element.style.display = 'none';
+    document.body.appendChild(element);
+    element.click();
+    document.body.removeChild(element);
+}
+
 function el(id) {
     return document.getElementById(id);
 }
@@ -35,6 +47,7 @@ async function run() {
     await init();
 
     const buttonAddImage         = el("add");
+    const buttonDownload         = el("download");
     const buttonGameDataProceed  = el("game-data-next");
     const textareaGameDataInput  = el("game-data");
     const buttonImportGame       = el("import");
@@ -110,6 +123,13 @@ async function run() {
     buttonImportGame.addEventListener("click",    addTiles);
     buttonImportGame.addEventListener("touchend", addTiles);
 
+    function handleDownload() {
+        download("output.bitsy", textareaGameDataOutput.value);
+    }
+
+    buttonDownload.addEventListener("click",    handleDownload);
+    buttonDownload.addEventListener("touchend", handleDownload);
+
     function addImage() {
         textareaGameDataInput.value = textareaGameDataOutput.value;
         textareaGameDataOutput.value = "";