Very simple string interpolation in JavaScript. {} expands to positional arguments, {NAME} to named arguments, {{ to {, and }} to }.

format.js

function format (fmt, kwargs) {
	var args = arguments;
	var index = 1;
	return fmt.replace(/{([^{}]*)}|{{|}}|{|}/g, function (match, key) {
		if (key !== undefined) {
			if (key) {
				return kwargs[key];
			}
			else {
				return args[index ++];
			}
		}

		switch (match) {
			case '{{': return '{';
			case '}}': return '}';
			case '{': throw new SyntaxError('Unmatched left curly bracket "{" in format.');
			case '}': throw new SyntaxError('Unmatched right curly bracket "}" in format.');
		}
	});
}