fix(wasm): work with service workers (#334)

BREAKING CHANGES:

This change leverages `globalThis` and will fail in older environments where `globalThis` is not implemented. See https://caniuse.com/#search=WebAssembly and https://caniuse.com/#search=globalThis. If your environment doesn't support `globalThis`, please use an earlier version of this plugin.

* Make wasm plugin work with service workers

The code generated by @rollup/plugin-wasm breaks when executed in a
service worker because window is undefined.

* Add test for worker scenario and simplify the fix

Instead of checking whether window is defined, we can just use
globalThis.

* Only run worker test if worker_threads supported

packages/wasm/src/index.js

@@ -30,7 +30,7 @@ export default function wasm(options = {}) {
         if (isNode) {
           buf = Buffer.from(src, 'base64')
         } else {
-          var raw = window.atob(src)
+          var raw = globalThis.atob(src)
           var rawLength = raw.length
           buf = new Uint8Array(new ArrayBuffer(rawLength))
           for(var i = 0; i < rawLength; i++) {

packages/wasm/test/fixtures/worker.js

@@ -0,0 +1,11 @@
+import sample from './sample.wasm';
+// atob doesn't exist in node, so we fake it
+// in the browser, globalThis.atob will exist
+// eslint-disable-next-line no-undef
+globalThis.atob = (str) => Buffer.from(str, 'base64').toString('binary');
+// trick the wasm loader into thinking we are in the browser
+const realProcess = process;
+// eslint-disable-next-line no-global-assign, no-undefined
+process = undefined;
+sample({});
+realProcess.exit(0);

packages/wasm/test/test.js

@@ -6,11 +6,15 @@ import wasm from '../dist/index';
 
 const AsyncFunction = Object.getPrototypeOf(async () => {}).constructor;
 
-const testBundle = async (t, bundle) => {
+const generateCode = async (bundle) => {
   const { output } = await bundle.generate({ format: 'cjs' });
   const [{ code }] = output;
-  const func = new AsyncFunction('t', `let result;\n\n${code}\n\nreturn result;`);
+  return code;
+};
 
+const testBundle = async (t, bundle) => {
+  const code = await generateCode(bundle);
+  const func = new AsyncFunction('t', `let result;\n\n${code}\n\nreturn result;`);
   return func(t);
 };
 
@@ -61,3 +65,29 @@ test('imports', async (t) => {
   });
   await testBundle(t, bundle);
 });
+
+try {
+  const { Worker } = require('worker_threads');
+  test('worker', async (t) => {
+    t.plan(2);
+
+    const bundle = await rollup({
+      input: 'test/fixtures/worker.js',
+      plugins: [wasm()]
+    });
+    const code = await generateCode(bundle);
+    const executeWorker = () => {
+      const worker = new Worker(code, { eval: true });
+      return new Promise((resolve, reject) => {
+        worker.on('error', (err) => reject(err));
+        worker.on('exit', (exitCode) => resolve(exitCode));
+      });
+    };
+    await t.notThrowsAsync(async () => {
+      const result = await executeWorker();
+      t.true(result === 0);
+    });
+  });
+} catch (err) {
+  // worker threads aren't fully supported in Node versions before 11.7.0
+}