Javascript add to string with each function call

Question

I have the following situation where I have a function f which takes an argument input.

I want to be able to have f such that

Answer 1

Try this

function f(input) {
  let str = 'f'
  var o = function(suffix){
    if(suffix)
      return str + suffix;
    str += "o";
    return o;
  }
  return o(input);
}

console.log(f('l'))
console.log(f()('l'))
console.log(f()()('l'))
console.log(f()()()('l'))