Grâce à Stack overflow (comme toujours):
window[klass]
<html>
<head>
</head>
<body>
<script>
function Toto()
{
this.pleinDeFonction = function()
{
};
}
Toto.f = function()
{
alert('tralala');
}
function Titi()
{
this.pleinDautresFonction = function()
{
};
}
Titi.f = function(){
alert('pouet pouet');
}
var klass = 'Toto';
var Qlass = window[klass];
Qlass.f(); // tralala
klass = 'Titi';
var Qlass = window[klass];
Qlass.f(); // pouet pouet
</script>
</body>
</html>