Insert character after every n characters

Write a function that takes two strings (a and b) as arguments. Start with the end of ‘a’, insert ‘b’ after every 3rd character of string ‘a’.

const insertChart = (str, c) => {
    let resultStr = '';
    let index = 0;
    for(let i = (str.length - 1); i > -1 ; i--) {
        index++;
        resultStr = str[i] + resultStr;
        if(index % 3 === 0) {
            resultStr = c + resultStr;
        }
    }
    return resultStr;
};

const input = 'zxyzxyzxyzxyzxyz';
const inputChar = '.';
console.log(insertChart(input, inputChar));