formatDuration.js

// formats a duration as a human readable string, ex. "5 days, 4 minutes, and 1 second ago".
function formatDuration(ms) {
	let parts = [];

	const addPart = (value, maxValue, label) => {
		value = Math.floor(value) % maxValue;
		if (value > 0) {
			parts.unshift(value + " " + label + (value === 1 ? "" : "s"));
		}
	}

	const seconds = ms / 1000;
	addPart(seconds, 60, "second");
	const minutes = seconds / 60;
	addPart(minutes, 60, "minute");
	const hours = minutes / 60;
	addPart(hours, 24, "hour");
	const days = hours / 24;
	addPart(days, Infinity, "day");

	let result = "";

	switch (parts.length) {
		case 0: {
			return "now";
		};

		case 1:
		case 2: {
			return parts.join(" and ") + " ago";
		};

		default: {
			parts.push("and " + parts.pop());
			return parts.join(", ") + " ago";
		}
	}
}

// reuse allowed with attribution