Array Question

onlynumbers.js

let array = [1, 2, 3, '4', undefined, 'a', [], null];

function onlyNumbers(arr) {
    let output = [];
    // If 'isNaN()' doesn't return true for the given element, we pushit to the output array
    arr.forEach(e => { if (!isNaN((e))) { output.push((e)) } })
    return output
}
console.log(onlyNumbers(array));

//  Wich outputs [ 1, 2, 3, '4', [], null];