Add NIP-40 module for event expiration

nip40.ts

@@ -0,0 +1,49 @@
+import { Event } from './core.ts'
+
+/** Get the expiration of the event as a `Date` object, if any. */
+function getExpiration(event: Event): Date | undefined {
+  const tag = event.tags.find(([name]) => name === 'expiration')
+  if (tag) {
+    return new Date(parseInt(tag[1]) * 1000)
+  }
+}
+
+/** Check if the event has expired. */
+function isEventExpired(event: Event): boolean {
+  const expiration = getExpiration(event)
+  if (expiration) {
+    return Date.now() > expiration.getTime()
+  } else {
+    return false
+  }
+}
+
+/** Returns a promise that resolves when the event expires. */
+async function waitForExpire(event: Event): Promise<Event> {
+  const expiration = getExpiration(event)
+  if (expiration) {
+    const diff = expiration.getTime() - Date.now()
+    if (diff > 0) {
+      await sleep(diff)
+      return event
+    } else {
+      return event
+    }
+  } else {
+    throw new Error('Event has no expiration')
+  }
+}
+
+/** Calls the callback when the event expires. */
+function onExpire(event: Event, callback: (event: Event) => void): void {
+  waitForExpire(event)
+    .then(callback)
+    .catch(() => {})
+}
+
+/** Resolves when the given number of milliseconds have elapsed. */
+function sleep(ms: number): Promise<void> {
+  return new Promise((resolve) => setTimeout(resolve, ms))
+}
+
+export { getExpiration, isEventExpired, waitForExpire, onExpire }