for await example

JavaScript/a-for-await.js

@@ -0,0 +1,38 @@
+'use strict';
+
+async function * slowCounter() {
+  let counter = 0;
+  while (true) {
+    counter += 1;
+    yield new Promise((resolve) => {
+      setTimeout(() => resolve(counter), 1000);
+    })
+  }
+}
+
+async function * fastCounterDown() {
+  let counter = 1000000;
+  while (true) {
+    counter -= 1;
+    yield counter;
+  }
+}
+
+async function* zip (a, b) {
+  while (true) {
+    const [first, second] = await Promise.all([
+      a.next(),
+      b.next(),
+    ]);
+
+    yield [first.value, second.value];
+  }
+}
+
+async function main () {
+  for await (let it of zip(slowCounter(), fastCounterDown())) {
+    console.log(it);
+  }
+}
+
+main();