Refactor hello-web example for possible future automated testing

For now there's a Node.js test that works, but we're not actually running it in CI,
and Node is not a dependency of the project.

examples/hello-web/index.html

@@ -3,10 +3,10 @@
     <div id="output"></div>
     <script src="platform/host.js"></script>
     <script>
-      roc_web_platform_run(
-        "./hello-world.wasm",
-        document.getElementById("output")
-      );
+      const elem = document.getElementById("output");
+      roc_web_platform_run("./hello-world.wasm", (string_from_roc) => {
+        elem.textContent = string_from_roc;
+      });
     </script>
   </body>
 </html>

examples/hello-web/platform/host.js

@@ -1,6 +1,4 @@
-const test_decoder = true;
-
-async function roc_web_platform_run(wasm_filename, dom_node) {
+async function roc_web_platform_run(wasm_filename, callback) {
   const decoder = new TextDecoder();
   let memory_bytes;
   let exit_code;
@@ -8,7 +6,7 @@ async function roc_web_platform_run(wasm_filename, dom_node) {
   function js_display_roc_string(str_bytes, str_len) {
     const utf8_bytes = memory_bytes.subarray(str_bytes, str_bytes + str_len);
     const js_string = decoder.decode(utf8_bytes);
-    dom_node.textContent = js_string;
+    callback(js_string);
   }
 
   const importObj = {
@@ -25,10 +23,17 @@ async function roc_web_platform_run(wasm_filename, dom_node) {
     },
   };
 
-  const wasm = await WebAssembly.instantiateStreaming(
-    fetch(wasm_filename),
-    importObj
-  );
+  let wasm;
+
+  const response = await fetch(wasm_filename);
+
+  if (WebAssembly.instantiateStreaming) {
+    // streaming API has better performance if available
+    wasm = await WebAssembly.instantiateStreaming(response, importObj);
+  } else {
+    const module_bytes = await response.arrayBuffer();
+    wasm = await WebAssembly.instantiate(module_bytes, importObj);
+  }
 
   memory_bytes = new Uint8Array(wasm.instance.exports.memory.buffer);
 
@@ -41,3 +46,9 @@ async function roc_web_platform_run(wasm_filename, dom_node) {
     }
   }
 }
+
+if (typeof module !== 'undefined') {
+  module.exports = {
+    roc_web_platform_run,
+  };
+}

examples/hello-web/test-node.js

@@ -0,0 +1,25 @@
+/**
+ * Node.js test file for hello-web example
+ * We are not running this in CI currently, and Node.js is not a Roc dependency.
+ * But if you happen to have it, you can run this.
+ */
+
+// Node doesn't have the fetch API
+const fs = require("fs/promises");
+global.fetch = (filename) =>
+  fs.readFile(filename).then((buffer) => ({
+    arrayBuffer() {
+      return buffer;
+    },
+  }));
+
+const { roc_web_platform_run } = require("./platform/host");
+
+roc_web_platform_run("./hello-world.wasm", (string_from_roc) => {
+  const expected = "Hello, World!";
+  if (string_from_roc !== expected) {
+    console.error(`Expected "${expected}", but got "${string_from_roc}"`);
+    process.exit(1);
+  }
+  console.log("OK");
+});