javascript - new MyFunction() vs. new(MyFunction) - Stack Overflow

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I'm looking through the Vows documentation and in several places it uses the syntax

var myVar = new(MyFunction);

e.g.

var promise = new(events.EventEmitter);

I am familiar with new MyFunction() and new MyFunction (and yes, I have read this question). But the syntax above is, well, new to me - it looks like a function call, though I suspect it's just new MyFunction with some added parentheses. Is there any difference between these ways of using new? If not, is there any good argument for using one or the other? I would have thought new MyFunction() was the most legible.

Apologies if this is a duplicate - I searched but couldn't find it.

I'm looking through the Vows documentation and in several places it uses the syntax

var myVar = new(MyFunction);

e.g.

var promise = new(events.EventEmitter);

I am familiar with new MyFunction() and new MyFunction (and yes, I have read this question). But the syntax above is, well, new to me - it looks like a function call, though I suspect it's just new MyFunction with some added parentheses. Is there any difference between these ways of using new? If not, is there any good argument for using one or the other? I would have thought new MyFunction() was the most legible.

Apologies if this is a duplicate - I searched but couldn't find it.

• javascript
• syntax
• new-operator

Share Improve this question Follow edited May 23, 2017 at 12:16 CommunityBot 111 silver badge asked Mar 27, 2012 at 4:45 nrabinowitznrabinowitz 55.7k1010 gold badges151151 silver badges169169 bronze badges

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3 Answers 3

Sorted by: Reset to default 4

They appear to be the same, just with parentheses in different places, perhaps stylistically or for clarity to the author(s). The following are all identical:

function Foo() { this.now = new Date(); }
var f1 = new Foo;
var f2 = (new Foo);
var f3 = new(Foo);
var f4 = new Foo();
var f5 = (new Foo());

Note that the form below is different because the "Foo" function is called directly (due to the precedence of the parens), returning nothing (undefined), which is an invalid argument to the "new" operator (because "undefined" is not a function):

var x = new(Foo()); // TypeError: undefined is not a function

if

var MyFunction = function(){
    alert("ok");
}

then

var myVar = new(MyFunction);

bees

var myVar = new(function(){
    alert("ok");
});

It is just a matter of organizing code.

The example you made is equivalent, there is no differences: the new is an operator. It's not unmon find parenthesis around operators like:

if (typeof(myvar) === "function")

or also:

return(myvar);

But they are operators, parenthesis are not needed. I don't like them in these contexts, but it's a matter of personal preferences.

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