fastArray.js

var generateArray=function(x,n)
{
    var y,z;
    y=" "+x; //added space here
    z="";
    while(n>0)
    {
        if(n%2)
        {
            z+=y;
            n--;
        }
        if(n===0)
        {
            break;
        }
        y+=y;
        n/=2;
    }
    //trimemd space here and split on it
    return z.trim().split(" ");
};

var generateArrayShorter=function(val,len)
{
    var out = new Array(len);
    for(var idx = 0; idx < out.length; idx++){
        out[idx]=val;
    }
    return out;
};

var generateArrayShortest=function(val,len)
{
    return new Array(len+1).join(val+' ').trim().split(' ');
};
 

var myArray1 = generateArray("x",1594323);
// Takes 124ms

var myArray2 = generateArrayShorter("x",1594323);
// Takes 162ms

var myArray3 = generateArrayShortest("x",1594323);
// Takes 229ms

console.log(myArray1);
//takes about 14 seconds