For instance we define a number of functions

function a () { return 0; }

function b () { return 1; }

function c () { return 2; }



var probas = [ 20, 70, 10 ]; // 20%, 70% and 10%

var funcs = [ a, b, c ]; // the functions array

That generic function works for any number of functions, it executes it and return the result:

function randexec()

{

  var ar = [];

  var i,sum = 0;





  // that following initialization loop could be done only once above that

  // randexec() function, we let it here for clarity



  for (i=0 ; i<probas.length-1 ; i++) // notice the '-1'

  {

    sum += (probas[i] / 100.0);

    ar[i] = sum;

  }





  // Then we get a random number and finds where it sits inside the probabilities 

  // defined earlier



  var r = Math.random(); // returns [0,1]



  for (i=0 ; i<ar.length && r>=ar[i] ; i++) ;



  // Finally execute the function and return its result



  return (funcs[i])();

}

For instance, let's try with our 3 functions, 100000 tries:

var count = [ 0, 0, 0 ];



for (var i=0 ; i<100000 ; i++)

{

  count[randexec()]++;

}



var s = '';

var f = [ a, b, c ];



for (var i=0 ; i<3 ; i++)

  s += (s ? ', ':'') + f[i] + ' = ' + count[i];



alert(s);

The result on my Firefox

a = 20039, b = 70055, c = 9906

So a run about 20%, b ~ 70% and c ~ 10%.

Edit following comments.

If your browser has a cough with return (funcs[i])();, just replace the funcs array

var funcs = [ a, b, c ]; // the old functions array

with this new one (strings)

var funcs = [ a, b, c ]; // the new functions array

then replace the final line of the function randexec()

return (funcs[i])(); // old

with that new one

return eval(funcs[i]+'()');