call stack error

똑같이 작성한것같은데 에러가 콜 스택 에러가 나는데 왜  그런걸까요 ...ㅠㅠ

function mergeSort(array){
let arrayLength = array.length;
let result = [];
if(arrayLength <= 1){
return array;
}

let mid = parseInt(array/2);
let firstGroup = mergeSort(array.slice(0,mid));
let secondGroup = mergeSort(array.slice(mid, ))

while(firstGroup.length != 0 && secondGroup.length !=0){
if(firstGroup[0]<secondGroup[0]){
    result.push(firstGroup.shift());
}else{
result.push(secondGroup.shift());
}
}

while(firstGroup.length != 0){
result.push(firstGroup.shift())
}

while(secondGroup.length != 0){
result.push(secondGroup.shift())
}

return result;
}