Question

My colleague has been using “new function()” with a lower case “f” to define new objects in JavaScript. It seems to work well in all major browsers and it also seems to be fairly effective at hiding private variables. Here’s an example:

    var someObj = new function () {
        var inner = 'some value';
        this.foo = 'blah';

        this.get_inner = function () {
            return inner;
        };

        this.set_inner = function (s) {
            inner = s;
        };
    };

As soon as “this” is used, it becomes a public property of someObj. So someObj.foo, someObj.get_inner() and someObj.set_inner() are all available publicly. In addition, set_inner() and get_inner() are privileged methods, so they have access to “inner” through closures.

However, I haven’t seen any reference to this technique anywhere. Even Douglas Crockford’s JSLint complains about it:

• weird construction. Delete ‘new’

We’re using this technique in production and it seems to be working well, but I’m a bit anxious about it because it’s not documented anywhere. Does anyone know if this is a valid technique?

Answer 1

I’ve seen that technique before, it’s valid, you are using a function expression as if it were a Constructor Function.

But IMHO, you can achieve the same with an auto-invoking function expression, I don’t really see the point of using the new operator in that way:

var someObj = (function () {
    var instance = {},
        inner = 'some value';

    instance.foo = 'blah';

    instance.get_inner = function () {
        return inner;
    };

    instance.set_inner = function (s) {
        inner = s;
    };

    return instance;
})();

The purpose of the new operator is to create new object instances, setting up the [[Prototype]] internal property, you can see how this is made by the [Construct] internal property.

The above code will produce an equivalent result.