When returning an object from a JavaScript constructor function (avoiding 'new') how do I implement public members?-Collection of common programming errors

Your problem is that you aren’t taking advantage of closure. The line id:id creates a new variable as a member of the object literal you are returning. This has the default value undefined. Then you are assigning the same variable id back to itself, undefined again.

Also, a module is a single object instance, not a factory function. Try something like this:

var myModule = (function(opts) {
    var foo = opts.foo || 0,    //private variables with default values
        bar = opts.bar || 0,    // if none are passed in

        product = function() {      //private method
            return (foo * bar);

    return {
        getFoo : function() {   //public methods
            return foo;
        fooTimesBar : function() {
            return product();
    foo : 5,                    //options object
    bar : 7

The core of the module pattern is a self executing anonymous function that declares variables and then returns an object that has privileged access to those variables through closure.

The extra parens at the end, here containing an object literal with some options that get passed in, executes the function, which then returns an object and assigns it to myModule.

Anything declared as a member of that returned object can be accessed publicly. Code in the returned object has access to the variables defined in the anonymous function through closure even after the function has returned. The variables declared in the anonymous function are effectively private. Code outside the function cannot address them except through the methods provided in the returned object.

The whole thing results in a single object instance in myObject. No more than one can be created, which is the definition of a module. A similar approach could be taken the create a factory function however.

Originally posted 2013-11-09 23:04:37.