Add throttle() helper.

Useful to avoid repeated updates triggered by
incoming row-level database events.

lib/www/client/source/src/lib/throttle.js

@@ -0,0 +1,33 @@
+/**
+ * Throttle a function call.
+ *
+ * It delays `callback` by `delay` ms and ignores any
+ * repeated calls from `caller` within at most `maxWait`
+ * milliseconds.
+ *
+ * Used to react to server events in cases where we get
+ * a separate notification for each row of a bulk update.
+ */
+function throttle (callback, caller, delay = 100, maxWait = 500) {
+
+  const schedule = async () => {
+    caller.triggeredAt = Date.now();
+    caller.timer = setTimeout(async () => {
+      await callback();
+      caller.timer = null;
+    }, delay);
+  }
+
+  if (!caller.timer) {
+    schedule();
+  } else {
+    const elapsed = Date.now() - caller.triggeredAt;
+    if (elapsed > maxWait) {
+      cancelTimeout(caller.timer);
+      schedule();
+    }
+  }
+
+}
+
+export default throttle;