flatten.js

function flattenArray(arr) {
    if (arr === undefined || arr === null) {
        throw new Error('Bad argument type:', arr);
    }
    const flattenedArray = [].concat(...arr)
    for(const m of flattenedArray) {
        if (m instanceof Array) {
            return flattenArray(flattenedArray);
        }
    }
    return flattenedArray;
}

// === Some test cases =====

// Complex case 1
console.log(flattenArray([[1,2,[3]],4]));

// Complex case 2
console.log(flattenArray([[1,2,[3]],4, [[[5]]]]));

// The empty array
console.log(flattenArray([]));

// Passing undefined as arg
console.log(flattenArray(undefined));

//Passing null
console.log(flattenArray(null));