Puzzle: JS Function that returns itself until there are no arguments

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I'm trying to solve a puzzle, and am at my wit's end trying to figure it out.

I'm supposed to make a function that works like this:

add(1);       //returns 1
add(1)(1);    //returns 2
add(1)(1)(1); //returns 3

I know it can be done because other people have successfully completed the puzzle. I have tried several different ways to do it. This is my most recent attempt:

function add(n) {
    //Return new add(n) on first call
    if (!(this instanceof add)) {
        return new add(n);   
    }

    //Define calc function
    var obj = this;
    obj.calc = function(n) {
        if (typeof n != "undefined") {
            obj.sum += n;
            return obj.calc;            
        }

        return obj.sum;
    }

    //Constructor initializes sum and returns calc(n)
    obj.sum = 0;
    return obj.calc(n);
}

The idea is that on the first call, a new add(n) is initialized and calc(n) is run. If calc receives a parameter, it adds n to sum and returns itself. When it eventually doesn't receive a parameter, it returns the value of sum.

It makes sense in theory, but I can't get it to work. Any ideas?

asked 1 min ago

Eclecticist

579

Puzzle: JS Function that returns itself until there are no arguments