I can't get eval() to work with my javascript calculation

Question

I am working on a javascript code which calculates the value of Pi. So here's the problem. When eval() calculates the string which is generated by the code it returns 1 . It is supposed to return 1.49138888889 (which is a rough value of Pi^2/6 ).

Here's the code. It returns the correct calculation as a string, But eval() doesn't calculate it properly.

function calculate() {
  var times = 10;
  var functionpart1 = "1/";
  var functionpart2 = "^2+";
  var x;
  for (var functionpistring = "", x = 1; times != 0; times--, x++) {
    functionpistring = functionpistring + functionpart1 + x.toString() + functionpart2;
  }
  document.getElementById("value").innerHTML = eval(functionpistring.slice(0, functionpistring.length - 1));
}

Answer 1

function calculate() {
    var times = 20, // max loops
        x,          // counter
        f = 0,      // value representation
        s = '';     // string representation
    function square(x) {
        return x * x;
    }
    function inv(x) {
        return 1 / x;
    }
    function squareS(x) {
        return x + '²';
    }
    function invS(x) {
        return '1 / ' + x;
    }
    for (x = 0; x < times; x++) {
        f += square(inv(x));
        s += (s.length ? ' + ' : '') + squareS(invS(x));
        document.write(f + ' = ' + s);
    }
}
calculate();