Evaluate the sum of all the 4 digit numbers that can be formed with the digits 1,3, 5 and 7. code example

Example: Write a function digitsum that calculates the digit sum of an integer. The digit sum of an integer is the sum of all its digits.

function digSum(n) {
    let sum = 0;
    let str = n.toString();
    console.log(parseInt(str.substring(0, 1)));
    for (let i = 0; i < str.length; i++) {
        sum += parseInt(str.substring(i,i+1));
        
    }
    return sum;
}