Examples written for my students.

filter.js

function filter(arr, func) {
    const res = [];
    
    for (const val of arr) {
        if (func(val)) {
            res.push(val);
        }
    }
    
    return res;
}

console.log(filter(
    [2, 4, 6, 8, 10],
    (currentValue) => {
        return currentValue > 5;
    }
));