Add countdown timer for gacha enthusiasts

static/countdown.js

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+// countdown timer for gacha enthusiasts
+// sample: bandori/wiki/BanG_Dream!_Wikia
+// sample: ensemble-stars/wiki/The_English_Ensemble_Stars_Wiki
+
+import {h, htm, render, useState, useEffect, createContext, useContext, signal, computed, effect} from "./preact.js"
+const html = htm.bind(h)
+
+const now = signal(Date.now())
+setInterval(() => now.value = Date.now(), 1000)
+
+const units = [
+	["w", 7*24*60*60*1000],
+	["d", 24*60*60*1000],
+	["h", 60*60*1000],
+	["m", 60*1000],
+	["s", 1000]
+]
+
+function getDisplayTime(datetime, now, or) {
+	let difference = datetime - now
+	let foundSignificantField = false
+	if (difference > 0) {
+		return units.map(([letter, duration], index) => {
+			const multiplier = Math.floor(difference / duration)
+			difference -= multiplier * duration
+			if (multiplier > 0 || foundSignificantField) {
+				foundSignificantField = true
+				return multiplier + letter
+			}
+		}).filter(s => s).join(" ")
+	} else if (or) {
+		return or
+	} else {
+		return `[timer ended on ${new Date(datetime).toLocaleString()}]`
+	}
+}
+
+function Countdown(props) {
+	return html`<span>${props.display}</span>`
+}
+
+document.querySelectorAll(".countdown").forEach(eCountdown => {
+	// grab information and make variables
+	const eDate = eCountdown.querySelector(".countdowndate")
+	const eOr = eCountdown.nextElementSibling
+	const or = eOr?.textContent
+	const datetime = new Date(eDate.textContent).getTime()
+	// the mapped signal
+	const display = computed(() => getDisplayTime(datetime, now.value, or))
+	// clear content and render
+	while (eDate.childNodes[0] !== undefined) eDate.childNodes[0].remove()
+	render(html`<${Countdown} display=${display} />`, eDate);
+})