MergeSort.js

function mergeSort(arr) {
    if (arr.length < 2) return arr;
    let mid = Math.ceil(arr.length / 2);
    let arrLeft = mergeSort(arr.splice(0, mid));
    let arrRight = mergeSort(arr.splice(-mid));
    return merge(arrLeft, arrRight);
}

function merge(arr1, arr2) {
    let arr = [];
    while (arr1.length && arr2.length) {
        arr.push(arr1[0] <= arr2[0] ? arr1.shift() : arr2.shift());
    }
    return [...arr, ...arr1, ...arr2];

}

arr = [3, 5, 6, 2, 1, 7, 4];
console.log(mergeSort(arr));