Code challenge for Theorem job application

theorem.js

/**
 * This function takes a multidimensional array and return a flat array by running recursively
 * @param arr
 * @returns {Array}
 */
const arr_flatten = (arr) => {
    //empty accumulator
    const flat = [];

    arr.forEach(item => {
        if (Array.isArray(item)) {
            //spread + recursion
            flat.push(...arr_flatten(item));
            return;
        }
        flat.push(item);
    });

    return flat;
};


/**
 * Auxiliar method for use on test cases
 * @param array
 * @param flat
 * @param i
 */
const method_assert = (array, flat, i) => {
    const testJSON = JSON.stringify(arr_flatten(array));
    const flatJSON = JSON.stringify(flat);
    if (testJSON !== flatJSON) {
        console.log(testJSON, ' Diff ', flatJSON);
        console.warn(`TEST ${i} fails`);
        return;
    }
    console.log(`TEST ${i} passes`)
};


/**
 * Run this function to test all the use cases
 */
const test = () => {
    //Multidimensional case
    method_assert([1, [2, 3, 4], [5, [6, [7, 8]]]], [1, 2, 3, 4, 5, 6, 7, 8], 1);
    //Multidimensional case with strings
    method_assert([1, [2, 3, ['a', 'b', ['c']]], [5, [6, [7, 8]]]], [1, 2, 3, 'a', 'b', 'c', 5, 6, 7, 8], 2);
    //Same array
    method_assert([1, 2, 3, 4], [1, 2, 3, 4], 3);
};