One way to flatten in ES6, not necessarily the best

flatten.js

function flatten(x) {
    var result = [];
    var appendToResult = function (xs) {
        result.splice(result.length, 0, ...xs);
    }
    if (Array.isArray(x)) {
        x.map(flatten).forEach(appendToResult);
    } else {
        result.push(x);
    }
    return result;
}