Using es6 spread to concat multiple arrays

Question

We all know you can do:

let arr1 = [1,2,3];
let arr2 = [3,4,5];
let arr3 = [...arr1, ...arr2]; // [1,2,3,3,4,5]

But how do you make this dynami

Answer 1

We can resolve using es6 following way

function mergeTwo(arr1, arr2) {
  let result = [...arr1, ...arr2];
  return result.sort((a,b) => a-b);
}

Answer 2

You could use a recursive function and Array.prototype.concat

const concatN = (x,...xs) =>
  x === undefined ? [] : x.concat(concatN(...xs))

console.log(concatN([1,2,3], [4,5,6], [7,8,9]))
// [1,2,3,4,5,6,7,8,9]

You can do the same thing using reduce and Array.prototype.concat. This is similar to the accepted answer but doesn't senselessly use spread syntax where x.concat(y) is perfectly acceptable (and likely heaps faster) in this case

const concatN = (...xs) =>
  xs.reduce((x,y) => x.concat(y), [])

console.log(concatN([1,2,3], [4,5,6], [7,8,9]))
// [1,2,3,4,5,6,7,8,9]

Answer 3

You can use spread element within for..of loop to concatenate array values to a single array

let arr1 = [1,2,3];
let arr2 = [3,4,5];
let arr3 = [];

for (let arr of [arr1, arr2 /* , arrN */]) arr3.push(...arr);

console.log(arr3);

Answer 4

One option is to use reduce:

let arrs = [[1, 2], [3, 4], [5, 6]];
arrs.reduce((a, b) => [...a, ...b], []);

Of course, this is a slow solution (quadratic time). Alternatively, if you can use Lodash, _.flatten does exactly what you want, and does it more efficiently (linear time).

EDIT

Or, adapted from Xotic750's comment below,

[].concat(...arrs);

Which should be efficient (linear time).

Answer 5

Another option could be:

const nArrays = [
  [1, 2, 3, 4, 5],
  [6, 7, 8, 9],
  [10, 11]
];
const flattened = [].concat(...nArrays);
console.log(flattened)

Answer 6

let fruits = ["apples", "bananas", "pears"];
let vegetables = ["corn", "potatoes", "carrots"];

let produce = [...fruits, ...vegetables];


console.log(produce);