flatten-multidimensional-array.js

function flatten(arr){
    if (!Array.isArray(arr)){
        return [arr];
    }
    return arr.reduce((flat, toFlatten) => {
        return flat.concat(flatten(toFlatten));
    }, []);
}

function test(objUnderTest){
    console.log(flatten(objUnderTest));
}

test(1);
test([1,2,3,4])
test([1,[2,3],[4,5],6,7,[[8],9]]);
test([1,[2,3],[4,5],6,7,[[8],[9,[10,[11,[12]]]]]]);