You can define classes in JavaScript, in a similar way as you do it in C#. The following example is a class that inherits from MonoBehaviour.

JavaScript:

class MyClass extends MonoBehaviour {
  var myVar = 1;
  function Start() {
    Debug.Log("Hello World!");
  }
}

C#:

class MyClass : MonoBehaviour {
  public int myVar = 1;
  void Start() {
    Debug.Log("Hello World!");
  }
}

However in JavaScript, if you're inheriting from MonoBehaviour, you don't need to write a class body at all. You can also write the following script in JavaScript, which will get a similar result as the preceding JavaScript:

var myVar = 1;
function Start() {
  Debug.Log("Hello World!");
}

Unity will automatically implement an explicit class body for you.

You can also write classes that do not inherit from anything; however, you can't place these scripts on the game objects—you have to instantiate them with the new keyword.

JavaScript:

class MyClass {
  var myVar = 1;
  function MyClass() {
    Debug.Log("Hello World!");
  }
}

C#:

class MyClass {
  public int myVar = 1;
  void MyClass() {
    Debug.Log("Hello World!");
  }
}