ECMAScript 2019 will introduce the Array.prototype.flat() method to aid in flattening out multidimensional arrays in JavaScript. However, flattening arrays has been easy in JavaScript every since ECMAScript 6 (2015) introduced generators. Because generators can yield values in any order, not just linearly, it’s possible to use a recursive generator to easily flatten an array. Here’s the code:

function* traverse(array) {
    for (const item of array) {
        if (Array.isArray(item)) {
            yield* traverse(item);
        } else {
            yield item;
        }
    }
}

const data = [
    1, 2,
    [
        3, 4,
        [5, 6]    
    ],
    7, 8
];

const flattened = [...traverse(data)];
console.log(flattened);         // 1,2,3,4,5,6,7,8

The generator just checks each item to see if it’s an array, and if it is, then it delegates the yield operation to a recursive traverse() call. In the simple case, when the array item isn’t an array, the item is yielded. This ouputs each item linearly, which means using a spread operator with the returned iterator results in a flattened array.