Array.prototype.depth

depth.js

// 配列の深さ(次元)を返す
Array.prototype.depth = function () {
	function isArray(a) {
		return typeof(a) === "object" && (a instanceof Array);
	};
	
	var arr = this;
	return (function(a, b) {
		a = a || arr;
		b = b || 0;
		if (isArray(a) && a.length) {
			b++;
			var c = [];
			for (var i=0,l=a.length; i<l; i++) {
				c[c.length] = arguments.callee(a[i], b);
			}
			return Math.max.apply(null, c);
		}
		return b;
	})();
};

// Usage
console.log([8,[[9]]].depth());
// -> 3