Flatten nested array of integers

flatten.js

/* I've chosen Javascript as it has become my main language nowadays. I tried
 to avoid high level functions () and stick to just the basic stuff.
 */
function flatten(object) {
    return object.reduce((a, b) => {
        if (Array.isArray(b)) {
            return a.concat(flatten(b));
        } else {
            return a.concat(b);
        }
    }, []);
}

/* Usually my Javascript scripts have this section at the end, which is
   the equivalent of
       if __name__ == '__main__'
   in Python */
if (require.main === module) {
    let exampleList = [[1,2,[3]],4];
    let flattenedList = flatten(exampleList);
    console.log('example list: ', exampleList);
    console.log('flattened list:', flattenedList);
}