flatten.js

function flatten(array) {
    let flattened = [];

    for (let i = 0; i < array.length; i++) {
        if (Array.isArray(array[i])) {
            flattened = flattened.concat(flatten(array[i]))
        } else {
            flattened.push(array[i])
        }
    }

    return flattened;
}