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

As I suggest you in comments, you should better use generators to get each value after each call.

function * f(input) {
    for(const character of input) {
        yield character;
    }
}
test = f('hello world');
test.next(); // {value: "h", done: false}
test.next(); // {value: "e", done: false}
test.next(); // {value: "l", done: false}
test.next(); // {value: "l", done: false}
test.next(); // {value: "o", done: false}
test.next(); // {value: " ", done: false}
test.next(); // {value: "w", done: false}
test.next(); // {value: "o", done: false}
test.next(); // {value: "r", done: false}
test.next(); // {value: "l", done: false}
test.next(); // {value: "d", done: false}
test.next(); // {value: undefined, done: true}

As this, you just have to call the next function each time you need and concatenate the result.value each time.

Here more docs about generators : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function