SOURCE

var div=document.getElementById('div');
var value=div.innerText;
var n=1;
var timer=null;
var index=300;
div.style.visibility='visible';

function fun(){
    
    var str=value
    str=str.substr(0,n);
    var len=value.length;
    
    timer=window.setTimeout(fun,index);
    div.innerHTML=str;

    n++;
    if(n==len+1) {
        n=0;
        window.clearTimeout(timer);
    }

}

window.onload=fun;

<div id="div" >
  1234567891012345678910123456789
</div>