What's the returned value of javascript constructor function

Example:

function Foo(){
return something;
}
var a = new Foo();

The exact condition is described on the

[[Construct]]

internal
property, which is used by the

new

operator:

From the ECMA-262 3rd. Ediion Specification:

13.2.2

[[Construct]]

When the

[[Construct]]

property
for a

Function

object

F

is
called, the following steps are taken:

Create a new native ECMAScript object.

Set the

[[Class]]

property
of

Result(1)

to

"Object"

.

Get the value of the prototype property of

F

.

If

Result(3)

is
an object, set the

[[Prototype]]

property
of

Result(1)

to

Result(3)

.

If

Result(3)

is
not an object, set the

[[Prototype]]

property
of

Result(1)

to
the original

Object

prototype
object as described in 15.2.3.1.

Invoke the

[[Call]]

property
of

F

,
providing

Result(1)

as
the

this

value
and providing the argument list passed into

[[Construct]]

as
the argument values.

If

Type(Result(6))

is

Object

then
return

Result(6)

.

Return

Result(1)

.

Look at the steps 7 and 8, the new object will returned only if the type of

Result(6)

(the
value returned from the

F

constructor
function) is not an Object.
Steps 7 “Type(x) is Object” means “x instance of Object is true”, not “typeof(x) === ‘object’ is true”

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