Flatten an array of nested arrays of integers

flatten.ts

const { expect } = require('chai');

/**
 * Flattens nested arrays
 * @param {Array} array The array to flatten.
 * @returns {Array} Returns the new flattened array.
 */
const flatten = (arr: Array<any>): Array<number> => arr.toString().split(',').map(x => parseInt(x, 10)).filter(x => !!x) || [];

describe('flatten', () => {
    it('should flatten nested arrays', () => {
        const arr = [[1,2,[3]],4];
        const result = flatten(arr);
        expect(result).to.have.members([1, 2, 3, 4]);        
    });

    it("ignores anything that's not a number", () => {
        const arr = [1, 2, ['string', 3]];
        const result = flatten(arr);
        expect(result).to.have.members([1, 2, 3]);
    });
});