diamond.js

/**
 * Star diamond
 */

String.prototype.repeat = function(n) {
	if(n < 0) throw new RangeError('Invalid count value');
	var ret = '';
	while(n--) ret += this;
	return ret;
};

function diamond(n) {
	if(n < 1 || n % 2 == 0) return null;
	
	var diamond = '';
	for(
		var
			line = (n - 1) / 2,
			boundary = -line - 1,
			indent;
		line > boundary;
		line--
	) {
		indent = Math.abs(line);
		diamond += ' '.repeat(indent) + '*'.repeat(n - 2 * indent) + '\n';
	}

	return diamond;
}