main.js

// Using commonjs js modules
import utils from './services/utils';
let array = [[1,2,[3]],4];
let newArray = utils.flat(array);
console.log(newArray);

utils.js

// utils.service
export default const utils = {
    flat(array) {
      let flatedArr = array.reduce(function iter(acc, item)  {
        if (Array.isArray(item)) {
          return item.reduce(iter, acc);
        } else {
          return acc.concat(item);
        }
      }, []);
      return flatedArr;  
    },
  
    otherUtilFcn() {
      
    }
}

utils.test.js

//Test done with jest

import utils from './utils';
describe('when need to flat array', () => {
    test('should flat normal array', () => {
        // Arrange
        const arr = [1,2,3,4,5];

        // Act
        const response = utils.flat(arr);
      
        // Assert
        expect(response).toEqual([1,2,3,4,5]);
    });
  
    test('should flat complex array', () => {
        // Arrange
        const arr = [1,2,3,4,5,[1,2], [1, [2,3,[4,5,[6,7]]]],8];

        // Act
        const response = utils.flat(arr);
      
        // Assert
        expect(response).toEqual([1, 2, 3, 4, 5, 1, 2, 1, 2, 3, 4, 5, 6, 7, 8]);
    });
  
});