Add runAsync snippet

snippets/runAsync.md

@@ -0,0 +1,53 @@
+### runAsync
+
+Runs a function in a separate thread by using a Web Worker, allowing
+long running functions to not block the UI.
+
+Create a new `Worker` using a `Blob` object URL, the contents of which should be the
+stringified version of the supplied function. Immediately post the return value of calling
+the function back. Return a promise, listening for `onmessage` and `onerror` events
+and resolving the data posted back from the worker, or throwing an error.
+
+```js
+const runAsync = fn => {
+  const blob = `
+    var fn = ${fn.toString()};
+    this.postMessage(fn());
+  `;
+  const worker = new Worker(
+    URL.createObjectURL(new Blob([blob]), {
+      type: 'application/javascript; charset=utf-8'
+    })
+  );
+  return new Promise((res, rej) => {
+    worker.onmessage = ({ data }) => {
+      res(data), worker.terminate();
+    };
+    worker.onerror = err => {
+      rej(err), worker.terminate();
+    };
+  });
+};
+```
+
+```js
+const longRunningFunction = () => {
+  let result = 0;
+  for (var i = 0; i < 1000; i++) {
+    for (var j = 0; j < 700; j++) {
+      for (var k = 0; k < 300; k++) {
+        result = result + i + j + k;
+      }
+    }
+  }
+  return result;
+};
+
+// NOTE: Since the function is running in a different context, closures are not supported.
+// The function supplied to `runAsync` gets stringified, so everything becomes literal.
+// All variables and functions must be defined inside.
+runAsync(longRunningFunction).then(console.log); // 209685000000
+runAsync(() => 10 ** 3).then(console.log); // 1000
+let outsideVariable = 50;
+runAsync(() => typeof outsideVariable).then(console.log); // 'undefined'
+```