Concat Arrays with For Loop - arrays

Question

After experimenting with the reduce method to flatten an array, I tried using a similar approach with a for-loop.
Can anyone explain why the for-loop doesn't flatten the array using concat?
P.S. I know that I can use a nested for-loop with Array.isArray to flatten as well. Just expected concat to work in a for-loop after seeing how reduce flattened the array.
var arrays = [
[1, 2, 3],
[4, 5],
[6]
];
console.log(arrays.reduce(function(arr, elem) {
return arr.concat(elem);
}, []));
function flatten(arr) {
var flat = [];
for (var i = 0; i < arr.length; i++) {
flat.concat(arr[i]);
}
return flat;
}
console.log(flatten(arrays));

Answer 1

concat doesn't modify the array. To make your code work, you'd have to assign the value to it, then it'll do the same
var arrays = [[1,2,3], [4,5], [6]];
function flatten(arr) {
var flat = [];
for (var i = 0; i < arr.length; i++) {
flat = flat.concat(arr[i]);
}
return flat;
}
console.log(flatten(arrays));
Just a sidenote - with ES6, you can flatten an array even easier using the spread operator [].concat(...arr);
var arrays = [[1,2,3], [4,5], [6]];
function flatten(arr) {
return [].concat(...arr);
}
console.log(flatten(arrays));
And nowadays even easier, using flat()
const arrays = [[1,2,3], [4,5], [6]];
console.log(arrays.flat());