求阶乘（参考:http://stackoverflow.com/questions/3959211/fast-factorial-function-in-javascript）

factorial-iterative.js

//Iterative
function factorial(num)
{
    var rval=1;
    for (var i = 2; i <= num; i++)
        rval = rval * i;
    return rval;
}

factorial-memoization.js

//memoization
var f = [];
function factorial (n) {
  if (n == 0 || n == 1)
    return 1;
  if (f[n] > 0)
    return f[n];
  else
    return f[n] = factorial(n-1) * n;
} ​

factorial-recursive.js

//Recursive
function factorial(num)
{
    if (num === 0)
      { return 1; }
    else
      { return num * factorial( num - 1 ); }
}