Getting length of array assigned to extended function prototype in Javascript

Question

The code below will log a, a.length, and b.test. a and b.test both yield [1, 2, 3]. Edit-- I screwed up. b.test yields undefined. See raina's response below.

a.length yields 3.

b.test.length fails with "Cannot read property 'length' of undefined"

Why is this the case when a and b.test are equal?

var a = [1,2,3];

var b = function(){};

b.prototype.test=[1,2,3];

console.log(a);

console.log(a.length);

console.log(b.test);

console.log(b.test.length);

Thanks, Matt. I'm seeing that too now. I must have overlooked something. — temporarylight, Dec 23 '12 at 18:02
Refer **[How does JavaScript .prototype work?](http://stackoverflow.com/a/572996/500725)** answer — Siva Charan, Dec 23 '12 at 18:13
possible duplicate of [How does JavaScript .prototype work?](http://stackoverflow.com/questions/572897/how-does-javascript-prototype-work) — bfavaretto, Dec 23 '12 at 21:02

score 5 · Accepted Answer · answered Dec 23 '12 at 17:55

Because b does not have test property - it's a function, and its prototype object is Function.prototype one.

But objects created with this function will have this property in their prototype chain (as prototype property of b points to the object that has test property defined, so this...

console.log(new b().test.length);

... should give you 3, as expected.

Getting length of array assigned to extended function prototype in Javascript

1 Answers1