为什么我的布尔值返回“未定义”？

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.

Answer 1

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