/** make text appear in the innerHTML of the element, one character at a time per timeBetween */
function appearChars(str, elem, timeBetween) {
    var index = -1;
    (function go() {
        if (++index < str.length) {
         	p.innerHTML = p.innerHTML + str.charAt(index);
            setTimeout(go, timeBetween);
        }
    })();
}

// example
var str = 'A wild text appears.';
var elem = document.getElementsByTagName('p')[0];
var timeBetween = 42;

appearChars(str, elem, timeBetween);