Flattening a JavaScript array with a generator

Generators make traversing multidimensional arrays easy.

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.