JavaScript new Function expression and new Function constructor

Question

My question is regarding the resulting object from using new function(){ ... } over new Function();

My understanding so far/assumption

When we create a function using Function() or new Function() we get a object configured as a function (it's internal slots indicate a function type object)
With or without the new operator, the Function() constructor returns a new function object
Using a function expression returns a function object which internally uses the Function() constructor
A function expression provides optimization with parsing the function body

My question

Following my assumption above, why does new Function(); and new function(){ ... } return different things?

The first returns a function object, but the latter returns a standard object. Given that a function expression under the hood uses the Function() constructor, why does the latter not behave the same as new Function();?

Using new function(){ ...} I would expect a function object, not a standard object.

function(){} == new Function(), new function () {} == new new Function(). — zzzzBov
– zzzzBov, Commented Jun 21, 2016 at 21:17

zzzzBov · Accepted Answer · 2016-06-22 02:11:02Z

The following lines are essentially the same:

function foo(...) {...}
var foo = function (...) {...};
var foo = new Function (...);

They all declare a new function (which is an object, that is an instance of Function, which inherits the Object prototype, ...) in the current scope which can be accessed via the foo variable.

The new keyword instantiates an object from a function which allows you to create something similar to a class instance in a standard OOP language.

Continuing the example from above:

var bar = new foo(...)

would instantiate an instance of the foo function in the current scope which could be accessed via the bar variable.

why does new Function(); and new function(){ ... } return different things?

I'm going to rephrase the example slightly. Instead of:

new Function()
//and
new function(){ ... }

I'm going to use

new Function()
//and
var foo = function () { ... };
new foo();

The reason that new Function() returns a different thing than new foo() is entirely because Function is a different function from foo.

I expect that you're not confused by the fact that new Object() returns a different object with different features than new Array(). The exact same thing is happening here.

Given that a function expression under the hood uses the Function() constructor, why does the latter not behave the same as new Function();?

under the hood the later expression is essentially:

new (new Function ( ... ))

Bálint · Accepted Answer · 2016-06-21 21:20:23Z

1

new Function(...) returns a new function, while new function() {...} returns a new Object.

Basically, Function is just a way to evaluate a piece of code.

Objects are a way, to store data in javascript.

Side note: you can technically do

new (new Function(...))

wich returns a new object.

answered Jun 21, 2016 at 21:20

Bálint

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