JSInt is an interactive JavaScript console. It may be useful to perform mathematical calculations or for learning JavaScript.
For example, you can try entering following statements ( user input is prefixed with > here ):
> x = 10
10
> x = x / 5
2
> Math.sin(Math.PI / x)
1
> f = function(x) { return x^2 + 3*x + 5}
f = function(x) { return x^2 + 3*x + 5}
> f(x)
15
JSInt uses JavaScriptCore Open Source library which implements JavaScript 1.5 / ECMA-262 3rd edition.