Calling a function without entering its parameters in Javascript

Question

I am trying to create a JavaScript version of an old timey dice game named PIG. It involves rolling dice and keeping track of the sum of each roll. I need to be able to save each roll within a function and then be able to call said function to return the current roll score.

This is the method that takes the amount of the current roll and stores it into the variable total, subsequently returning the current total variable.

  function rollTotal(amount) {
  return amount;
  }

I first call the function and insert the score of the dice roll into the amount parameter...

var dieOne = 1;
var dieTwo = 2;
rollTotal(dieOne + dieTwo);

Then I call the function to return the current score like this...

rollTotal()

and am getting this...

undefined

Is there a way to set a default parameter to return the current score if a parameter is not entered when called?

This doesn't make sense. rollTotal doesn't update any variable. It's just an identity function, so rollTotal() returns undefined instead of NaN. — Oriol
– Oriol, Commented Sep 6, 2015 at 0:37
@Oriol Thanks for the heads up! That is indeed what I was getting, I just posted a typo. I corrected my error. — Erik Åsland
– Erik Åsland, Commented Sep 6, 2015 at 19:16

Ted Hopp · Accepted Answer · 2015-09-06 01:00:07Z

1

When you call a function and don't supply a parameter, then the parameter is bound to undefined within the function. So you can do something like this:

var currentAmount = undefined; // or perhaps 0?
function rollTotal(amount) {
    if (typeof(amount) === 'undefined') {
        return currentAmount;
    } else {
        currentAmount = amount;
        return amount;
    }
}

You can do this more elegantly (and more safely) using a closure:

var rollTotal = (function() {
    var currentAmount = undefined;
    return function(amount) {
        if (typeof(amount) === 'undefined') {
            return currentAmount;
        } else {
            currentAmount = amount;
            return amount;
        }
    };
}());

You might also want to do a more robust test of the argument and treat any non-number as undefined. You can do this with the following test:

!isNaN(parseFloat(amount)) && isFinite(amount)

instead of testing specifically for undefined.

EDIT: As @Paul S. points out, the best test as to whether an argument was passed is arguments.length === 0.

edited Sep 6, 2015 at 1:00

answered Sep 6, 2015 at 0:38

Ted Hopp

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4 Comments

Ted Hopp Over a year ago

Would the downvoters care to explain what's wrong here? Are you reading something in OP's question that I'm missing?

Paul S. Over a year ago

A different test you could do is arguments.length === 0, otherwise the IIFE version of this is basically what I would've written

Ted Hopp Over a year ago

@PaulS. - Yes, testing arguments.length is probably the right way to go.

Erik Åsland Over a year ago

@TedHopp - Thanks! This will work just fine. I forgot about this option. You get the answer.

Paul S. · Accepted Answer · 2015-09-06 01:08:09Z

1

Why use a function? The = operator works just fine.

// initial setup
var rollTotal;

// set in the middle of an expression
"You just rolled " + (rollTotal = dieOne + dieTwo) + "!";

// recalling later in an expression
"Last time you rolled " + rollTotal + ", what will you get this time?";

answered Sep 6, 2015 at 1:08

Paul S.

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Comments

jczimm · Accepted Answer · 2015-09-06 00:57:20Z

0

A popular way to do this is with an instance of a class.

That is:

function Dice() {
    var amount;

    this.rollTotal = function(_amount) {
        // if an argument is supplied, assign it to the internal amount
        if (typeof _amount !== "number") {
            amount = _amount;
        }
        return amount;
    };
}

Example usage:

var dice = new Dice();
dice.rollTotal(3);
console.log(dice.rollTotal());

edited Sep 6, 2015 at 0:57

answered Sep 6, 2015 at 0:40

jczimm

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1 Comment

jczimm Over a year ago

Would the recent downvoter care to explain their reasoning?

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