Efficient algorithm to get the combinations of all items in object

Question

Given an array or object with n keys, I need to find all combinations with length x.
Given X is variable. binomial_coefficient(n,x)

Answer 1

And here's the true recursion.

function seq(a,b){
  var res = [];
  for (var i=a; i<=b; i++)
    res.push(i);
  return res;
}

function f(n,k){
  if (k === 0)
    return [[]];
    
  if (n === k)
    return [seq(1,n)];
    
  let left = f(n - 1, k - 1),
      right = f(n - 1, k);
    
  for (let i=0; i