Permutation algorithm in javascript

Perm.js

function swap(str, i, j) {
  let temp = str[i]
  
  str = str.split("");
  
  str[i] = str[j];
  str[j] = temp;
  
  return str.join("");
}
              
function perm(str, startIndex,  perms) {

  for(let i = startIndex; i < str.length; i++) {
    let temp = swap(str, startIndex, i);
    if(startIndex != i) perms.push(temp);
    
    perm(temp, startIndex + 1, perms);
  }
  
  return perms;
}

/* 

let str = "abcd"

console.log(perm(str, 0, [str]));

*/