Change prototype to new object: undefined

JavaScript
1

Hi,

Can anyone explain why my print to the console returns undefined with the following code:

function Dog(name) {
  this.name = name;
}

let terrier = new Dog("John");

Dog.prototype = {
  constructor: Dog,
  numLegs: 4
};

console.log(terrier.numLegs);

I would expect it to work like this does:

function Dog(name) {
  this.name = name;
}

let terrier = new Dog("John");

Dog.prototype.numLegs = 4;

console.log(terrier.numLegs);

My understanding was setting prototype to a new object was a quick way than setting each property individually, unless I’ve missed something?

Many thanks,

Fraser

2

when you define terrier, Dog.prototype doesn’t have the numLegs property

you need to create terrier after you finish setting up stuff for Dog

1 Like
3

Try this after both examples and see if it clears things a bit:

console.log(terrier.__proto__ === Dog.prototype);
1 Like
4