onlynumbers.js

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

function onlyNumbers(arr) {
    let output = [];
    //  Now we try to parseInt() each element, 
    //  and push a new Number() created with e as an argument.
    arr.forEach(e => { if (parseInt(e)) { output.push(Number(e)) } })
    return output
}
console.log(onlyNumbers(array));

// Wich outputs [ 1, 2, 3, 4 ]