Flatten an array of nested elements.

interview.js

// Flattens the given nested elements.
function flattenArray(elements) {
    if (elements === undefined || !Array.isArray(elements)) {
        throw "Array is expected";
    }
    return elements.reduce((result, current) => {
        return result.concat(Array.isArray(current) ? flattenArray(current) : current);
    }, []);
}

// Checks if two array are equal.
function assertArraysEqual(arr1, arr2) {
    if (arr1.length !== arr2.length) {
        return false;
    }
    for (var i = 0; i < arr1.length; i++) {
        if (arr1[i] !== arr2[i]) {
            return false;
        }
    }
    return true;
}

function assert(condition, message) {
    if (!condition) {
        message = message || "Assertion failed";
        if (typeof Error !== "undefined") {
            throw new Error(message);
        }
        throw message; // Fallback
    }
}

function testFlattenArray() {
    assert(assertArraysEqual(flattenArray([2, 2, 3]), flattenArray([[1], 2, 3])) === false);
    assert(assertArraysEqual(flattenArray([1, 2, 3]), flattenArray([[1], 2, 3])) === true);
    assert(assertArraysEqual(flattenArray([1, 2, 3]), flattenArray([1, 2, 3])) === true);
    assert(assertArraysEqual(flattenArray([]), flattenArray([])) === true);
    assert(assertArraysEqual(flattenArray([1, [2], [3]]), flattenArray([[1], 2, 3])) === true);
    assert(assertArraysEqual(flattenArray([[[1]], [[2]], 3]), flattenArray([[1], 2, 3])) === true);
    assert(assertArraysEqual(flattenArray([2, 2]), flattenArray([[1], 2, 3])) === false);
    assert(assertArraysEqual(flattenArray([2]), flattenArray([1])) === false);
}