const arr = [1, 2, 3, [4, 5, [6, 7, [8, 9, 10]]]]


const flattenArray = (arr)=>{
    let newArray =[]
    const flatten = (arg)=>{
        arg.forEach((item,index)=>{
            if(Array.isArray(item)){
                flatten(item);
            }
            else{
                newArray = [...newArray,item]
            }
        })
    };
    flatten(arr);
    return newArray;
}

const res = flattenArray(arr);
console.log(res);