mergeSort

function mergeSort(arr) {
if (arr.length < 2) {
return arr;
}
let mid = Math.floor(arr.length / 2);
let leftArr = arr.slice(0, mid);
let rightArr = arr.slice(mid);
return merge(leftArr, rightArr);
}

function merge(leftArr, rightArr) {
let sortedArr = [];
while (leftArr.length && rightArr.length) {
if (leftArr[0] < rightArr[0]) {
sortedArr.push(leftArr.shift());
} else {
sortedArr.push(rightArr.shift());
}
}
return [...sortedArr, ...leftArr ,...rightArr];
}

console.log(mergeSort([-2, -6, 8, 2, 5, 20, 1]));