JS function: create 2d array or multidimensional matrix w/o constructor

create-matrix.js

// create 2d array or multidimensional matrix
function createMatrix() {
	var matrix = [];
	matrix.add = function(val, i1, i2/*, ...*/) {
		var cur = matrix, curI;

		// loop from 2nd argument to 2nd to last
		for (var i = 1; i < arguments.length - 1; i++) {
			curI = arguments[i];
			
			// add an empty array in the current array of the matrix at every index
			// before and including the current index until a defined value is reached
			for (var j = curI; j >= 0 && !(curI in cur); j--) {
				cur[j] = [];
			}
			cur = cur[curI];
		}
		curI = arguments[i];
		cur[curI] = val;
			
		// add an empty array in the current array of the matrix at every
		// index before the added value until another defined value is reached
		for (var k = curI - 1; k >= 0 && !(curI in cur); k--) {
			cur[k] = [];
		}
	};
	return matrix;
}