function that flattens an array in javascript

flatten.js

function flatten(items) {
    return items.reduce((result, current) => {
        if (current.constructor === Array)
            return result.concat(flatten(current))
    
        return result.concat(current);
    }, []);
}


describe('flatten', () => {
    it('should ignore array with no elements', () => {
        expect(flatten([])).toEqual([]);
    });

    it('should ignore array with no nested arrays', () => {
        expect(flatten(['a','b','c'])).toEqual(['a','b','c']);
    });

    it('should expand nested arrays', () => {
        expect(flatten(['a',['b1','b2','b3'],'c'])).toEqual(['a','b1','b2','b3','c']);
        expect(flatten([[1,2,[3]],4])).toEqual([1,2,3,4]);
    });

    it('should expand multiple levels of nested arrays', () => {
        expect(flatten(['a',['b1','b2',['b31','b32']],'c'])).toEqual(['a','b1','b2','b31','b32','c']);
    });
});