Where to declare class constants?

Question

I\'m using class members to hold constants. E.g.:

function Foo() {
}

Foo.CONSTANT1 = 1;
Foo.CONSTANT2 = 2;

This works fine, except that it see

Answer 1

First, I recommend moving your class declaration inside of an IIFE. This cleans up the code, making it more self-contained, and allows you to use local variables without polluting the global namespace. Your code becomes:

var Foo = (function() {
  function Foo() {
  }

  Foo.CONSTANT1 = 1;
  Foo.CONSTANT2 = 2;

  return Foo;
})();

The problem with assigning constants directly to the class as attributes is that those are writable. See this snippet:

var output = document.getElementById("output");

var Foo = (function() {
  function Foo() {
  }

  Foo.CONSTANT1 = 1;
  Foo.CONSTANT2 = 2;

  return Foo;
})();

Foo.CONSTANT1 = "I'm not very constant";

output.innerHTML = Foo.CONSTANT1;

The best solution I have found is to define read-only properties for accessing the constants outside of the class.

var output = document.getElementById("output");

var Foo = (function() {
  const CONSTANT1 = "I'm very constant";

  function Foo() {
  }

  Object.defineProperty(Foo, "CONSTANT1", {
    get: function() {
      return CONSTANT1;
    },
  });

  return Foo;
})();

Foo.CONSTANT1 = "some other value";

output.innerHTML = Foo.CONSTANT1;

(Technically you could ditch the const CONSTANT1 statement and just return the value from the property definition, but I prefer this because it makes it easier to see all the constants at a glance.)