selection in a loop freezes my page

i'm a complete newbie when it comes to Js, trying to make some very simple script that takes a string of binary numbers from a txt document on my server using ajax, to then put it in a string var and change the first 0 it finds in a 1, using an if construct inside a loop.

Problem is, when the page tries to execute the if line, it simply freezes. Taking the same if construct out of the loop, the script is executed no problem, so i'm guessing it has something to do with either that or/and some fundamental misunderstanding of how Js scripts works in the first place.

Here is the script:

    function loadPos() 
    {                           
        var xhttp = new XMLHttpRequest();           
        xhttp.onreadystatechange = function() 
        {
            if (this.readyState == 4 && this.status == 200) 
                document.getElementById("demo").innerHTML = this.responseText;
        };
        xhttp.open("GET", "posizioni.txt", true);
        xhttp.send();
    }      

    function takeFirst()
    {
        var i=0;
        var check=false;
        var oldPos=[];

        loadPos();
        oldPos = document.getElementById("demo").innerHTML;

        for(i=0;!check||i<10;i++)
        {
            if(oldPos[i]=="0")
            {
                oldPos[i]="1";
                check=true;
            }
        }

        document.getElementById("demo").innerHTML=oldPos;
    }