jslib/lib/array/flatten.js

import checkArray  from './_checkArray';

export default (ar) => {
    checkArray(ar);

    return ar.reduce((acc, item) => {
        if (Array.isArray(item)) {
            acc.push(...flatten(item));
        } else {
            acc.push(item);
        }

        return acc;
    }, []);
};