flatten.js

/* 
It does a reduce, where it will test if each item is an array, 
if it is it will call the function recursively, 
it's possible only because of array.concat which will merge arrays together.
or you could use the spread sintax to merge them together IE: [...acc, ...flatten(item)], 
but I think this way it's a little more readble.
*/
const flatten = arrays => arrays.reduce((acc, item) => Array.isArray(item) ? acc.concat(flatten(item)) : acc.concat(item), []);

describe('Flatten arrays', () => {
    it('should flatten [1,2,3,[4,5,6, [7,8,9]],10,[11,[12],13]], to [1,2,3,4,5,6,7,8,9,10,11,12,13]', () => {
        const input = [1,2,3,[4,5,6, [7,8,9]],10,[11,[12],13]];
        const result = [1,2,3,4,5,6,7,8,9,10,11,12,13];
        expect(flatten(input)).toEqual(result)
    });

    it('should ignore empty spaces [1,2,3,[4,[], 5,6, [[[[[],[],[]]]]]], [7,8,9]], to [1,2,3,4,5,6,7,8,9]', () => {
        const input = [1,2,3,[4,[], 5,6, [[[[[],[],[]]]]], [7,8,9]]];
        const result = [1,2,3,4,5,6,7,8,9];
        expect(flatten(input)).toEqual(result)
    });    
});