Flatten an array

flatten.js

//Flatten array
const flatten = (arr) => [].concat.apply([], arr);

//To use 
const arrToFlatten = [
  ["1"],
  ["2"],
  ["3"],
  ["4"]
];
flatten(arrToFlatten);

// Test 
import assert from "assert"

describe('Should flatten array', function() {
  
  it('should flatten array', function() {
    assert.equal([[1], [2], [3]], [1,2,3]);
  });
  
  it('should flatten empty array', function() {
    assert.equal([], []);
  });
  
  it('should flatten already flattened array', function() {
    assert.equal([1,2,3], [1,2,3]);
  });
  
});