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Question

I am having trouble seeing where I have gone wrong in writing this 'transformArray' function. I am trying to make it take in a function as an argument, apply it to each element of some array. Why does it not return anything?

var array1 = [1,2,3,4,5];
function transformArray (aFunction) {
    return function (array) {
        return array.forEach(aFunction);
    };
}
var halve = transformArray(function (num) {return num/2;});
console.log(halve(array1));

Answer 1

That's because Array.forEach is an iterator. It always returns undefined .

MDN: forEach() executes the callback function once for each array element; unlike every() and some() it, always returns the value undefined .

What you need is Array.map

The map() method creates a new array with the results of calling a provided function on every element in this array.

function transformArray (aFunction) {
    return function(array) {
        return array.map(aFunction);
    };
}

(Emphasis mine)