Javascript function to transform string to add character upfront - 1 21 321 code example

Example: Javascript function to transform string to add character upfront - 1 21 321

function triangleNumber(num) {
    if(num>15) return;
    var str = '';
    for(var i = num; i >= 1 ; i--)  str += i;
    console.log(str);
    triangleNumber(num-1)
}

triangleNumber(1);