JS array flatten polyfill

.js

const assert = require('assert');

function flatten(a) {
    if (!Array.isArray(a)) throw new Error('Input value should be array');
    let stack = JSON.parse(JSON.stringify(a))
    let result = [];
    while (stack.length) {
        let a = stack.pop();
        if (Array.isArray(a)) {
            stack.push(...a);
        } else {
            result.push(a);
        }
    }
    return result.reverse();
}

console.log('Starting tests 🤖');

assert.deepEqual(flatten([1, 2, 3, [4, 5, [3]]]), [1, 2, 3, 4, 5, 3], 'Array should be flatten');
assert.notDeepEqual(flatten([1, 2, 3, [4, 5, [3]]]), [3, 5, 4, 3, 2, 1], 'Order of array should be correct');
assert.deepEqual(flatten([]), [], 'Should works correctly with empty array');
assert.throws(() => flatten('incorrect input'), Error, 'Should throws error for incorrect input');

console.log('Tests finished successfully 🎉');