keeping the constructor correctly after inheritance

I've noticed this interesting problem:

function a() { this.aprop = 1; }
function b() { this.bprop = 2; }
b.prototype = new a(); // b inherits from a
var开发者_StackOverflow x = new b(); // create new object with the b constructor
assert(x.constructor == b); // false
assert(x.constructor == a); // true

As far as I know, x.constructor should be b, but it's actually a when b inherits from a through its prototype? Is there a way I can inherit from a without screwing up my constructor?

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This is because b.prototype.constructor is assigned new a().constructor on the 3rd line. You can change this property back on the following line:

function a() { this.aprop = 1; }
function b() { this.bprop = 2; }
b.prototype = new a(); // b inherits from a
b.prototype.constructor = b; // <-- add this
var x = new b(); // create new object with the b constructor
assert(x.constructor == b); // false
assert(x.constructor == a); // true