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

f() can not return fo, it has to return a function. To get fo you need to call f like f()('').

Try this:

function f(input, acc) {
    acc = acc || 'f'
    if (input === undefined) {
        return function(a) {
            return f(a, acc + 'o')
        }
    }
    return acc + input
}

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