Why does my boolean return 'undefined'?

Question

I'm working through Eloquent Javascript and there's an exercise to make an every function, which takes an array and a function and returns true or false depending on what all the items in the array return after going through the function.

I'm confused because when I do console.log() within the function, I get the boolean twice...but when I do console.log(every(arr, func)), I get undefined.

var every = function(arr, req){
    arr.map(function(item){
        return req(item);
    }).reduce(
        function(total, num){

            // this returns: true
            //               true
            console.log(total && num);

            return total && num;
    });


}

// This returns undefined
console.log(every([NaN, NaN, NaN], isNaN));

So why do I get true twice inside my function, and why do I get undefined?

I'm using node as a console.

you never returned anything from every, so console.log(every(...)) will always be undefined. — user400654
– user400654, Commented Sep 18, 2015 at 21:26
The missing return aside, you really should just use arr.every(req), and if you go for reduce nonetheless you should always pass an initial accumulator value to cover empty arrays. — Bergi
– Bergi, Commented Sep 19, 2015 at 14:38

Daniel Schmidt · Accepted Answer · 2015-09-19 14:27:08Z

2

You need to add a return statement to your outermost function, like this:

var every = function(arr, req){
    return arr.map(function(item){
        return req(item);
    }).reduce(
        function(total, num){ 
            return total && num;
    });
}

// This returns true
console.log(every([NaN, NaN, NaN], isNaN));

EDIT: Fixed return value, thanks @Kevin B

edited Sep 19, 2015 at 14:27

answered Sep 18, 2015 at 21:29

Daniel Schmidt

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1 Comment

Daniel Schmidt Over a year ago

@KevinB absolutely, my bad.

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