Custom map function in javascript

//implementing custom map function

function double(num){

    return num*2;

}

function customMap(arr,callback,index,length){

    var newArr = [];

    for(var i=0; i<length; i++){

        newArr[index] = callback(arr[i]);

        index++;

    }

    return newArr;

}

var arr=[1,2,45,10,5,6,7,8,9];

var doubled = customMap(arr,double,0,arr.length);

console.log(doubled);

//Native Map Function

var newArr = arr.map((item)=>{

    return item*2;

})

console.log(newArr);

Output:

[ 2, 4, 90, 20, 10, 12, 14, 16, 18 ] [ 2, 4, 90, 20, 10, 12, 14, 16, 18 ]