Why does this anonymous function return undefined in Javascript?

Question

I'm trying to create a forEach() function that takes an array and a function, then performs the function action on each element of the array. However, when trying to pass the below anonymous function, I get undefined. I tried adding a return to the forEach() function after reading some other posts but then the function doesn't run at all and just returns the first array[i] it receives without modifying it.

function forEach(array, action){
 for(var i = 0; i < array.length; i++)
   action(array[i]);
}

var myArray = [1, 2, 3];
var something = forEach(myArray, function(element){return element++;});
console.log(something)
//undefined

That returns undefined.

function forEach(array, action){
 for(var i = 0; i < array.length; i++)
   return action(array[i]);
}

var myArray = [1, 2, 3];
var something = forEach(myArray, function(element){return element++;});
console.log(something)
//undefined

That returns 1.

What am I missing?

(I am aware a .forEach() function exists, I am trying this as a learning exercise)

First function doesn't have a return statement. Second one has a loop that exits on first iteration. It's a strange learning exercise anyway. — Álvaro González
– Álvaro González, Commented Jan 26, 2016 at 17:58
@ÁlvaroGonzález he is trying to implement forEach him self. It's a perfectly valid learning exercise I think. — Willem D'Haeseleer
– Willem D'Haeseleer, Commented Jan 26, 2016 at 18:00

Mike Cluck · Accepted Answer · 2016-01-26 18:07:35Z

3

Your forEach function doesn't return anything. It looks like you're trying your hand at a map implementation. If so, you need to add your results to a new array and return that array.

function map(array, action) { // Renamed forEach to a more suitable name
  var results = [];
  for (var i = 0; i < array.length; i++) {
    results.push(action(array[i]));
  }
  return results;
}

edited Jan 26, 2016 at 18:07

answered Jan 26, 2016 at 17:57

Mike Cluck

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1 Comment

dandavis Over a year ago

don't name a returning function "forEach", name it "map" to avoid confusion

georg · Accepted Answer · 2016-01-31 02:42:28Z

Your foreach implementation is correct - it shouldn't return anything. If you want map (i.e. transform an argument into a new array), you can implement reduce first:

function reduce(array, action, result) {
    forEach(array, function (elem) {
        result = action(result, elem);
    });
    return result;
}

and then, define map via reduce:

function map(array, action) {
    return reduce(array, function (result, elem) {
        return result.concat(action(elem));
    }, []);
}

Putting it all together:

function forEach(array, action) {
    for (var i = 0; i < array.length; i++)
        action(array[i]);
}

function reduce(array, action, result) {
    forEach(array, function (elem) {
        result = action(result, elem);
    });
    return result;
}

function map(array, action) {
    return reduce(array, function (result, elem) {
        return result.concat(action(elem));
    }, []);
}

a = map([1, 2, 3, 4], function (x) {
    return ++x
});
document.write('<pre>' + JSON.stringify(a, 0, 3));

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