Question

When making the prototypal inheritance, it’s asked to refer the child’s constructor back to itself below,

A = function() {} 
B = function() {} 
B.prototype = new A;
B.prototype.constructor = B;

What would take adverse effect if not?

EDIT

• As @GGG & @CMS have explained, the constructor alignment takes no
  effect on creating the child object by new Child(...), but is
  necessary to correctly reflect the child object’s constructor later.
• @GGG has also suggested a defensive alternative to extend
  the prototype chain. While Child.prototype = new Parent(...)
  includes parent’s properties to child, Child.prototype =
Object.create(Parent.prototype) doesn’t.

Answer 1

First, please don’t do this:

B.prototype = new A;

Do this instead (shim Object.create for old browsers):

B.prototype = Object.create(A.prototype);

As for constructor, nothing will break if you don’t do this, but if you don’t:

A = function() {};
var a = new A();
console.log(a.constructor); // A

B = function() {};
var b = new B();
console.log(b.constructor); // A (!)

…setting the constructor property of the prototype back to the actual constructor function allows you to do this:

B.prototype.constructor = B;
var b = new B();
console.log(b.constructor); // B