What values can a constructor return to avoid returning this?

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

From the ECMA-262 3rd. Edition Specification:

13.2.2 [[Construct]]

When the [[Construct]] property for a Function object F is
called, the following steps are taken:

  1. Create a new native ECMAScript object.
  2. Set the [[Class]] property of Result(1) to "Object".
  3. Get the value of the prototype property of F.
  4. If Result(3) is an object, set the [[Prototype]] property of Result(1) to Result(3).
  5. 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.
  6. 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.
  7. If Type(Result(6)) is
    Object then return Result(6).
  8. Return Result(1).

Look at steps 7 and 8, the new object will be returned only if the
type of Result(6) (the value returned from the F constructor
function) is not an Object.

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