Array Flattener

flattener.js

function flattenArray(nestedArray) {
    const flatArr = [];

    function flattenerFunc (arr) {
        arr.forEach(el => {
            if (Array.isArray(el)) {
                flattenerFunc(el);
            } else {
                flatArr.push(el);
            }
        });
    }

    flattenerFunc(nestedArray);

    return flatArr
}

module.exports = flattenArray;

flattener.spec.js

const flattenArray = require("./flattener");

test('flattens integer array', () => {
   const nestedArr = [0,[1,2,3,[4,[5]]],6,7];
   const flatteredArr = flattenArray(nestedArr);
   expect(flatteredArr).toEqual([0,1,2,3,4,5,6,7]);
});

test('flattens any array', () => {
    const anyNestedArr = ['a', [0, undefined], {name: "hello"}, [false, 5, {hello: "world"}]];
    const flatteredArr = flattenArray(anyNestedArr);
    expect(flatteredArr).toEqual(['a', 0, undefined, {name: "hello"}, false, 5, {hello: "world"}]);
});

Requires jest. Works not only with numbers, but with any type.