How can “new new Something” produce valid results in JavaScript?

Question

I\'m currently developing a JavaScript parser and study the ECMAScript 5.1 specification. Here\'s a question which puzzles me at the moment.

§ 11.2 Left-Hand

Answer 1

Take a look at section 13.2.2 [[Construct]] in the specification. More precisely, step 9 of the algorithm.

9. If Type(result) is Object then return result.

Functions are objects, so if you return a function from a function and try to instantiate the latter function using new, you'll get back the former function, not a new object. But functions are also constructors, so you can new the returned function.

function bar() {}
function foo() { return bar; }
new foo === bar; // true

Answer 2

It is not common at all, but it is possible; a function that returns a function:

function baz(){}
function foo(){return baz}

new new foo() instanceof baz // true

Answer 3

I had to use new new when using an object as namespace. Though there will be a nicer way:

Instead of:

var Namespace = function() {
    var ClassFirst = this.ClassFirst = function() {
        this.abc = 123;
    }
    var ClassSecond = this.ClassSecond = function() {
        console.log("Cluttered way to access another class in namespace: ", new new Namespace().ClassFirst().abc);
        console.log("Nicer way to access a class in same namespace: ", new ClassFirst().abc);
    }
}

new new Namespace().ClassSecond()

Do this:

var Namespace = new function() {
    var ClassFirst = this.ClassFirst = function() {
        this.abc = 123;
    }
    var ClassSecond = this.ClassSecond = function() {
        console.log("Cluttered way to access another class in namespace: ", new Namespace.ClassFirst().abc);
        console.log("Nicer way to access a class in same namespace: ", new ClassFirst().abc);
    }
}

new Namespace.ClassSecond()

Not directly related to the question, but I was googling new new javascript, because it looked pretty ugly and wrong. This post can help to avoid this unneeded object creation for fellow new new Google'ers.