---
id: 587d7dae367417b2b2512b7c
title: Use Prototype Properties to Reduce Duplicate Code
challengeType: 1
---
## Description
Since numLegs will probably have the same value for all instances of Bird, you essentially have a duplicated variable numLegs inside each Bird instance.
This may not be an issue when there are only two instances, but imagine if there are millions of instances. That would be a lot of duplicated variables.
A better way is to use Bird’s prototype. The prototype is an object that is shared among ALL instances of Bird. Here's how to add numLegs to the Bird prototype:
Bird.prototype.numLegs = 2;
Now all instances of Bird have the numLegs property.
console.log(duck.numLegs); // prints 2
console.log(canary.numLegs); // prints 2
Since all instances automatically have the properties on the prototype, think of a prototype as a "recipe" for creating objects.
Note that the prototype for duck and canary is part of the Bird constructor as Bird.prototype. Nearly every object in JavaScript has a prototype property which is part of the constructor function that created it.
## Instructions
Add a numLegs property to the prototype of Dog
## Tests
```yml
tests:
- text: beagle should have a numLegs property.
testString: assert(beagle.numLegs !== undefined, 'beagle should have a numLegs property.');
- text: beagle.numLegs should be a number.
testString: assert(typeof(beagle.numLegs) === 'number' , 'beagle.numLegs should be a number.');
- text: numLegs should be a prototype property not an own property.
testString: assert(beagle.hasOwnProperty('numLegs') === false, 'numLegs should be a prototype property not an own property.');
```
## Challenge Seed
```js
function Dog(name) {
this.name = name;
}
// Add your code above this line
let beagle = new Dog("Snoopy");
```
## Solution
```js
function Dog (name) {
this.name = name;
}
Dog.prototype.numLegs = 4;
let beagle = new Dog("Snoopy");
```