simple-phrase-formatter.js

const phrases = {
	"new_messages": "You have 1 new message",
	"new_messages_plural": "You have {{count}} new messages"
}

function formatPhrase(id, vars = {}) {
	// If there's a count, then we
	// we need to select the plural variant
	if (
		typeof vars.count != "undefined" &&
		vars.count != 1 &&
		!id.match(/_plural$/)
	) {
		return formatPhrase(`${id}_plural`, vars);
	}

	// replace vars with the actual values
	return Object.entries(vars).reduce(
		(p, [key, value]) => p.replace("{{"+key+"}}", value),
		phrases[id]
	);
}

console.log(formatPhrase("new_messages", { count: 0 }));
console.log(formatPhrase("new_messages", { count: 1 }));
console.log(formatPhrase("new_messages", { count: 2 }));

// You have 0 new messages
// You have 1 new message
// You have 2 new messages