How do you define an OOP class in JavaScript?

JavaScript is Prototype based and not class based.

Prototype-based programming is a style of object-oriented programming in which classes are not present, and behavior reuse (known as inheritance in class-based languages) is performed via a process of cloning existing objects that serve as prototypes. This model can also be known as class-less, prototype-oriented or instance-based programming. Delegation is the language feature that supports prototype-based programming.

I recommend this book for a concise, precise explanation of both how to use JS's prototypal inheritance as well as how to emulate classical OO inheritance in JS.

Any function in javascript can be used to create an object:

Example:

function MyPoint(x, y) {
    this.x = x;
    this.y = y;
    this.distanceTo = getDistance;
}

function getDistance(p) {
  var dx = this.x-p.x;
  var dy = this.y-p.y;
  return Math.sqrt(dx*dx + dy*dy);
}

var p0 = new MyPoint(1, 2);
var p1 = new MyPoint(2, 3);

window.alert('The distance is ' + p0.distanceTo(p1));