function insertChAfterEveryNCh(str : string, ch : string, n : number) : string {
for (let i = n; i < str.length; i += n + 1) {
str = str.slice(0, i) + ch + str.slice(i);
}
return str;
}
let str = 'javascript java typescript c python';
let N = 4;
str = insertChAfterEveryNCh(str, '*', N);
console.log(str);
/*
run:
"java*scri*pt j*ava *type*scri*pt c* pyt*hon"
*/