if (isNaN(solveString.innerHTML.charAt(solveString.innerHTML.length - 1)) &&
solveString.innerHTML.charAt(solveString.innerHTML.length - 1) != target.innerHTML) {
  solveString.innerHTML = solveString.innerHTML.substring(0, solveString.
    innerHTML.length - 1);
}

var html = solveString.innerHTML;
if (isNaN(html.charAt(html.length - 1)) && html.charAt(html.length - 1) != target.innerHTML){
    solveString.innerHTML = html.substring(0, html.length - 1)
}

var html = solveString.innerHTML;
var charAt = html.charAt(html.length - 1)
if (isNaN(charAt) && charAt != target.innerHTML){
    solveString.innerHTML = html.substring(0, html.length - 1)
}