How can I create an instance of another class from a different file

Question

I have searched all over for a comprehensible answer to this very simple question and cannot seem to find one. As a primarily java programmer, this has been a very frustrating process.

For example, say I am trying to program a deck of cards. The way I would do this would be to have a 'Card' class in card.js which would look something like this:

function Card(value, suit){
  this.value = value;
  this.suit = suit;

}

and then a 'Deck' class in deck.js which would look something like this :

function Deck(){
  this.cardArray = [];
  this.topCard = new Card(2, 'clubs');
}

Deck.prototype.shuffle = function(){
    //shuffle the deck
}

The problem here is that I get an error saying 'Unexpected Identifier'. Presumably because js doesn't realize that I have defined the Card class. How can I make it so the deck.js file can access the Card class?

I should mention that I am doing attempting to do this without a browser, so I think I would then be using node.js (Again, sorry I'm new to this environment). Or perhaps better stated, this will be server side.

If you are using node.js, then you probably want to look into the module system: nodejs.org/api/modules.html — UnholySheep
– UnholySheep, Commented Jul 19, 2017 at 18:09
FWIW, Deck.prototype.shuffle = function(){} should not be inside the constructor. — Felix Kling
– Felix Kling, Commented Jul 19, 2017 at 18:12
'Unexpected Identifier' is a syntax error. The code you posted doesn't throw that error, so you seem to have omitted the part that contains that error. To be clear: If you are not importing Card into wherever Deck is defined then you will still get an error, but that would be a different error. — Felix Kling
– Felix Kling, Commented Jul 19, 2017 at 18:14
@FelixKling shoot my bad, I was typing this up quickly, i'll edit it. Thanks! — Travis Pavletich
– Travis Pavletich, Commented Jul 19, 2017 at 18:14

Johnny John · Accepted Answer · 2017-07-19 18:15:25Z

1

You can use the module system

In your card.js:

const Card = function(value, suit){
 this.value = value;
 this.suit = suit;
}
module.exports = Card;

And in your deck.js

const Card = require('./card');

function Deck(){
 this.cardArray = [];
 this.topCard = new Card(2, 'clubs');

}

 Deck.prototype.shuffle = function(){
   //shuffle the deck
 }

edited Jul 19, 2017 at 18:15

answered Jul 19, 2017 at 18:12

Johnny John

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1 Comment

lleon Over a year ago

You should not use that arrow function. "this" is not referencing the instance.

KevBot · Accepted Answer · 2017-07-19 18:15:49Z

1

You need to use exports and imports:

In the card file, you would do:

function Card(num, suit) {
    this.num = num;
    this.suit = suit
}

module.exports = Card;

And then in the Deck file, you would do:

var Card = require('./Card.js');

function Deck() {
    this.cardArray = [];
    this.topCard = new Card(2, 'clubs');
}

Deck.prototype.shuffle = function () {
    //shuffle the deck
};

edited Jul 19, 2017 at 18:15

answered Jul 19, 2017 at 18:10

KevBot

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4 Comments

Travis Pavletich Over a year ago

It's giving me an error saying "Uncaught ReferenceError: module is not defined". Do I have to define it somewhere first?

KevBot Over a year ago

@TravisPavletich Are you using node to run this? Something like $ node Deck.js

Travis Pavletich Over a year ago

yes I am. I found the problem I just mentioned and fixed it. But it is still giving me an error. It says on the line this.topCard = new Card(2, 'clubs'); Syntax error: Unexpected Identifier.

Felix Kling Over a year ago

@TravisPavletich: That line should not produce this error. Do you have other code above it than what you posted here? Otherwise, the only explanation would be that the line contains an invalid non-printable character.

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