forEach returns undefined - How to return an array instead?

forEach doesn't return anything - use map instead:

var modifyArray = nums => nums.map(element => element % 2 == 0 ? element *= 2 : element *= 3) console.log(modifyArray([1,2,3,4,5]));

Also note that inside of the function passed to forEach you weren't returning anything - with ES5 function you need to explicitly return - not required with ES6 =>.

See the answers above.

There is a workaround if you still want to use forEach

let result = [] var modifyArray = (nums) => { nums.forEach(function (element) { result.push((element % 2 == 0) ? element *= 2 : element *= 3); }) return result; } modifyArray([1,2,3,4,5]) console.log(result) 

What about using for loops?

var modifyArray = (nums) => { for (let i = 0; i < nums.length; i++) { let element = nums[i]; // console.log(element + ' - ' + element % 2 ); nums[i] = (element % 2 == 0) ? element *= 2 : element *= 3; } return nums; } console.log(modifyArray([1,2,3,4,5])); 

For each returns undefined as it doesn't return anything but you can use forEach for doing the same thing by pushing your data into an array. Notice I did not change your approach but just extended it

var modifyArray = (nums) => { var somearray = [] nums.forEach((element) => somearray.push((element % 2 == 0) ? element *= 2 : element *= 3)); return somearray; } console.log(modifyArray([1, 2, 3, 4, 5]));

Use array.push to push data to an array. If you want to use the map method you can also use that like

var modifyArray = (nums) => { return nums.map((element) => { return (element % 2 == 0) ? element *= 2 : element *= 3 }); } console.log(modifyArray([1,2,3,4,5]));

Wanna use for loops? Then you can also try this approach. It's the same as the forEach loop

var modifyArray = (nums) => { var somearray = []; for (let i = 0; i < nums.length; i++) { somearray.push((nums[i] % 2 == 0) ? nums[i] *= 2 : nums[i] *= 3) } return somearray; } console.log(modifyArray([1, 2, 3, 4, 5]));