What's the returned value of javascript constructor function

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本文深入解析JavaScript中构造函数的工作原理及使用方式。通过ECMAScript规范详细解释了使用new操作符创建实例的过程，并强调了返回值类型对于实例化的影响。

Example:

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

The exact condition is described on the [[Construct]] internal property, which is used by the newoperator:

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”

Add one website for refer http://dmitrysoshnikov.com/ecmascript/javascript-the-core/