Javascript模块模式未捕获参考错误

Question

I've been following this tutoral, and when referencing Module as an argument in ModuleTwo, everything works fine until I comment out Module .

My understanding is that the double pipes || and empty object {} will create an empty object in place of Module if it's undefined, but instead I'm getting an error in the console.

var Module = (function () {

  var privateMethod = function () {
    // private
  };

  var someMethod = function () {
    // public
  };

  var anotherMethod = function () {
    // public
  };

  return {
    someMethod: someMethod,
    anotherMethod: anotherMethod
  };

})();

var ModuleTwo = (function (Module) {

    Module.extension = function () {
        // another method!
    };

    return Module;

})(Module || {});

Answer 1

When you apply the function defining your 2nd module to (Module || {}) , the symbol Module cannot be resolved if the Module hasn't been declared earlier, which always gives a JavaScript error. If you want the 2nd Module to be defined even in the absence of the first Module, try the following:

var ModuleTwo = (function(Module) {
   ...
   })(typeof Module == 'object' ? Module : {} ); 

Answer 2

Basically, there's an error in the tutorial. One way to make things work was suggested by rplantiko, but it might be easier to just write window.Module || {} window.Module || {} instead of Module || {} Module || {} .

How things work here:

• Accessing a non-existent property of any object yields undefined . However, referencing a variable that hasn't been declared yields ReferenceError (so your understanding was a little bit off there).
• Browser puts all global variables as properties onto global window object. Module in the tutorial is a global variable, because it's declared outside all functions, so you can access it via window.Module , which will not cause ReferenceError if undefined (per previous point).

It might be a good practice to explicitly assign to window any global variable you define (eg, do window.Module = (function () { … if you intend to make Module global), but that's arguable and out of scope of this discussion.