Js 3 kinds of merge operation instance code

The first:

function merge(a, b) {
    var aLen = a.length,
        bLen = b.length,
        maxLen = Math.max(aLen, bLen),
        sumLen = aLen + bLen,
        result = [],
        ap = 0,
        bp = 0;
     while (result.length < sumLen) {
        if (ap < aLen && bp < bLen) {
            if(a[ap] > b[bp]){
                result.push(b[bp++]);
            } else {
                result.push(a[ap++]);
            }
        } else if (!(ap < aLen)){
            while(bp < bLen){
                result.push(b[bp++]);
            }
        } else if (!(bp < bLen)){
            while(ap < aLen){
                result.push(a[ap++]);
            }
        }
    }
    return result;
}

The second:

function merge(arr1, arr2){

    var i = 0;
    var j = 0;
    var c = 0;
    var k;
    var len1 = arr1.length;
    var len2 = arr2.length;
    var arr = [];
    for(;i<len1 && j<len2;){
        if( arr1[i] > arr2[j]  ){
            arr.push( arr2[j] );
            j++;
        }else{
            arr.push( arr1[i] );
            i++;
        }
        //if(i==len1 || j==len2){
        //    break;
        //}
    }

    if(i==len1){
        //arr = arr.concat(arr2.slice(j));

        for(k=j; k<len2; k++){
            arr.push( arr2[k] );
        }
    }

    if(j==len2){
        //arr = arr.concat(arr1.slice(i))
        for(k=i; k<len1; k++){
            arr.push( arr1[k] );
        }

    }

    return arr;
}

The third:

function merge(a,b){
    var x = 0;
    var l = 0;
    var list = [];
    var aLen = a.length;
    var bLen = b.length;

    for(var i = 0; i < bLen; i++){
        for(var j = x; j < aLen; j++){
            if(b[i] < a[j]){
                list.push(b[i]);
                l = i;
                break;
            }else{                
                list.push(a[j]);
                x++;
            }
        }     
    }            
    if(x == a.length){
        for(var y = l; y < bLen; y++){
            list.push(b[y]);
        }
    }else{
        for(var z = x; z < aLen; z++){
            list.push(a[z]);
        }
    }

    return list;
}

After testing, the merge time of two ordered 20W arrays is less than 15 milliseconds.

The following are a few lessons (in a large number of operations to reflect, usually do not need to do this optimization. Code readability or first rule)

1: the concat method of arrays is slower than the direct for loop push.

2: for loop is faster than while loop.

3: var a = b| | 3; // this kind of operation takes a lot of time

4: break,continue is time consuming when it is determined that recirculation is not required.