Javascript Chainable Closure

Question

Inspired by If Hemingway Wrote Javascript, I'm trying to write a chainable function closure with a private local variable.

The intended behavior is:

> chainableCounter()
0
> chainableCounter(1)(2)()
3

Here's my code:

function chainableCounter(n) {
    var acc = 0;

    var fn = function (x) {
        if (x === undefined) {
            return acc;
        } else {
            acc = acc + x;
            return fn;
        }
    };

    fn(n);
}

When I try to run this in the node REPL, this is what I get:

> chainableCounter()
undefined
> chainableCounter(1)
undefined
> chainableCounter(1)()
TypeError: undefined is not a function

That chainableCounter(1) returns undefined instead of a function object seems to indicates that we're never hitting the line return fn;. But more than that, why does chainableCounter() also returning undefined when it should return 0? What am I missing?

Answer 1

fn(n) returns to the point of the call. Your actual function chainableCounter doesn't return anything, it doesn't have a return statement.

You can just add it before calling fn, like this:

return fn(n);