Level 5: Async and functions · Exercise 61 of 76 · Free

A counter with private state

A JavaScript exercise from Scriptdrill. Solve it with a code editor in your browser that runs your function against example and hidden tests, and it is checked the moment you submit.

The task

Return a counter object with three methods. `next()` returns the current value and then moves the counter on by `step`. `peek()` returns the current value without changing it. `reset()` puts the counter back to `start`. So `makeCounter()` gives 0, 1, 2 on successive `next()` calls. `step` can be negative or 0. Every counter is independent of the others.

What this exercise teaches

The methods are closures: they keep access to the variables of the makeCounter call that created them, and nothing outside can reach those variables. Each call to makeCounter creates a fresh set of variables, which is why two counters do not disturb each other.

A hint if you get stuck

Keep the current value in a variable inside `makeCounter` and return an object whose methods read and change that variable. Use default parameters for `start` and `step` rather than `||`, which would replace a step of 0.