Add CommonJS/Node/AMD snippet to end of compiled output

[rtfeldman]

See elm/compiler#1029 for discussion that led to this.

The following should be incorporated into the output JS of compiled Elm files.

(function() {
  var Elm = ... // current emitted code goes here

  if (typeof define === "function" && define.amd) {
    define([], function() {
      return Elm;
    });
  } else if (typeof module === "object") {
    module.exports = Elm;
  } else {
    if (typeof this.Elm === "undefined") {
      this.Elm = Elm;
    } else {
      throw new Error("This page is trying to import multiple compiled Elm programs using the same `Elm` global object, which would cause conflicts. This can be resolved by using a module loader like RequireJS to import the compiled Elm programs into different objects.")
    }
  }
}).call(this);

PREVIOUS VERSION:

This version is more "stock" and will silently overwrite any previous Elm globals defined on the page...which seems far less desirable than crashing early with a helpful message.

(function() {
  var Elm = ... // current emitted code goes here

  if (typeof define === "function" && define.amd) {
    define([], function() {
      return Elm;
    });
  } else if (typeof module === "object") {
    module.exports = Elm;
  } else {
    this.Elm = Elm;
  }
}).call(this);

[rtfeldman]

Just updated the original description to handle a case we ran into: if you add multiple compiled Elm programs to the same page, bad things will happen. The updated snippet will now:

1. Prevent that from happening
2. Explain to the user why that would be bad
3. Recommend an alternative

[itrelease]

#63

[rtfeldman]

Awesome! 🎉

[evancz]

Here is a version that uses an "early return" style that I think is easier to read.

    if (typeof define === "function" && define.amd)
    {
        define([], function() { return Elm; });
        return;
    }

    if (typeof module === "object")
    {
        module.exports = Elm;
        return;
    }

    if (typeof this.Elm === "undefined")
    {
        this.Elm = Elm;
        return;
    }

    throw new Error("blah blah blah");

}).call(this);

No need to do a PR or anything. I'll do it in the appropriate way.