How do I make a JavaScript (processing.js) class on KhanAcademy?

In the coding section of Khanacademy, I'm making a project in JavaScript (which is modified Processing.js here), and it gives errors for both notations of classes:

function Foo(bar) {
    this.bar = bar;
}

Foo.prototype.baz = function() { }

and

class Foo {

constructor(bar) {
    this.bar = bar;
}

baz() { }

The top one seemed mostly compatible, but the code wouldn't compile, saying "If you want to define a function, you should use 'var Foo = function() {};' instead".

Does anyone know how to make a class on this platform?

Khan Academy is good for learning the basics, but like you said it uses a modified version of Processing.js, which means that it chokes on syntax that should be valid.

If you really really need to use a class in Khan Academy, after playing around a bit, this example seems to work:

var Foo = function(x) {
    this.x = x;

    this.printX = function(){
      println('x: ' + this.x);  
    };
};

draw = function() {
  var myFoo = new Foo(42);
  myFoo.printX();
};

But you're probably better off using something like P5.js or even vanilla JavaScript.