Code Explanation Video Link :

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function mergeArray(arr1,arr2){
    let result=[];
    let i=0;
    let j=0;
    while(i<arr1.length && j<arr2.length){
        if(arr1[i]<arr2[j]){
            result.push(arr1[i]);
            i++;
        }
        else{
            result.push(arr2[j]);
            j++;
        }
    }
    while(i<arr1.length){
        result.push(arr1[i]);
        i++;
    }
    while(j<arr2.length){
        result.push(arr2[j]);
        j++;
    }
    return result;
 }

function mergeSort(array){
    if(array.length<=1)
        return array;
    let mid   = Math.floor(array.length/2);
    let left  = mergeSort(array.slice(0,mid));
    let right = mergeSort(array.slice(mid));
    return mergeArray(left,right);
}
mergeSort([6,3,2,9,1,23,65,34]);