add a generic webworker file

Makefile

@@ -57,7 +57,8 @@ all: build/pyodide.asm.js \
 	build/renderedhtml.css \
   build/test.data \
   build/packages.json \
-  build/test.html
+  build/test.html \
+  build/webworker.js
 
 
 build/pyodide.asm.js: src/main.bc src/jsimport.bc src/jsproxy.bc src/js2python.bc \
@@ -112,6 +113,8 @@ build/test.html: src/test.html
 build/renderedhtml.css: src/renderedhtml.less
 	lessc $< $@
 
+build/webworker.js: src/webworker.js
+	cp $< $@
 
 test: all
 	pytest test/ -v

src/webworker.js

@@ -0,0 +1,23 @@
+importScripts('./pyodide.js')
+
+var onmessage = function(e) { // eslint-disable-line no-unused-vars
+  languagePluginLoader.then(() => {
+    const data = e.data;
+    const keys = Object.keys(data);
+    for (let key of keys) {
+      if (key !== 'python') {
+        // Keys other than python must be arguments for the python script.
+        // Set them on self, so that `from js import key` works.
+        self[key] = data[key];
+      }
+    }
+    self.pyodide.runPythonAsync(data.python, () => {}).then((results) => {
+      self.postMessage({results});
+    }).catch((err) => {
+      // if you prefer messages with the error
+      self.postMessage({error: err.message});
+      // if you prefer onerror events
+      // setTimeout(() => { throw err; });
+    });
+  });
+}