如何在 Array.reduce() 的回调 function 中使用 Array.concat() 方法减少或展平 arrays 的数组

Question

//Bonus - uncomment lines 15 and 17
const arrays = [["how", "now"], ["brown", "cow"]];
const flattenedArray = arrays.reduce((a,c) => a + c);
// The below line should console.log: ["how", "now", "brown", "cow"]
console.log(flattenedArray);

I'm new to using the reduce function and it's a little bit complicated.

I'm trying to flatten the nested array but I don't really know what to do next.

Answer 1

You mentioned the solution already, you just need to implement it - concat the current item to the accumulator inside the reduce callback:

 const arrays = [["how", "now"], ["brown", "cow"]]; const flattenedArray = arrays.reduce((a,c) => a.concat(c)); console.log(flattenedArray);

But .flat() would be much easier:

 const arrays = [["how", "now"], ["brown", "cow"]]; const flattenedArray = arrays.flat(); console.log(flattenedArray);

Answer 2

Another option - flatMap:

 const arrays = [["how", "now"], ["brown", "cow"]]; const flattenedArray = arrays.flatMap(a => a); console.log(flattenedArray);

But it is only really advantageous if you want to do something inside the map, like this:

 const arrays = [["how", "now"], ["brown", "cow"]]; const flattenedArray = arrays.flatMap(a => a.concat(a)); console.log(flattenedArray);

Or like this:

 const arrays = [["how", "now"], ["brown", "cow"]]; const flattenedArray = arrays.flatMap(a => a.map(b => b.toUpperCase())); console.log(flattenedArray);