[ynherb]

Еще 2 варианта

function gn (arr, result = []) {
	var [j, mas]= [arr[0].shift(), arr.slice(1).join().split(',')]
	for(var i = 0; i < mas.length; i++) result.push(j+','+mas[i]);
	return (!arr[0].length) ? result : gn(arr, result);
}

console.log(gn( [[1,2,3], [4,5,6],[7,8,9]] ))

function gn (arr, result = []) {
	var [j, mas]= [arr[0].shift(), arr.slice(1)]
	for(var i = 0; i < mas.length; i++) 
		for(var k = 0; k < mas[i].length; k++) 
			(result[k] == undefined) ? result[k] = j+','+mas[i][k] : result[k] = result[k] + ','+ mas[i][k];
	return (!arr[0].length) ? result : result.concat(gn(arr));
}


console.log(gn( [[1,2,3], [4,5,6],[7,8,9]] ))