如果不返回任何函数，为什么函数返回未定义？ Java脚本

Question

I have a function like so

var tellMeTrueOrFalse = function(collection, callback){
    each(collection, function(item){ 
            console.log((callback(item))?true:false);
        } 
    );
};

( each is a function that behaves like underscore's _.each ). But when I try to console.log() the following:

tellMeTrueOrFalse([1,2,3], function(x) {return x%2 == 0;})

I get

false
true
false
undefined

Why am I getting undefined? when I'm not trying to return anything? How can I get it to just print and not returned the undefined ?

Answer 1

Why am I getting undefined? when I'm not trying to return anything?

you are getting undefined because a function will always return something. If there is no explicit return statement, then default value is returned. And default value of a function is undefined as per mozilla docs

To return a value other than the default, a function must have a return statement that specifies the value to return. A function without a return statement will return a default value. In the case of a constructor called with the new keyword, the default value is the value of its this parameter. For all other functions, the default return value is undefined.