Adding a method to a class using .prototype

Question

I am attempting to define a class called "User"... then further along in the code I am trying to add a method to the class by writing to "prototype". I'm not sure if my terminology is correct here... though I would like to have the method "who_auto" available to all future instances of "User"...

Trying out this code in JSFiddle... is giving me the error message: "Uncaught TypeError: pp.who_auto is not a function"

Here's my code:

class User {
  constructor(name) {
    this.name = name;
    this.chatroom = null;
  }

  who() {
    return `I am ${this.name}`;
  }
}

User.prototype = {
  who_auto: function() {
    console.log(`
  Hello, I am ${this.name}
  `);
  }
}
const pp = new User('peter parker');
console.log(pp);
console.log(pp.who());

pp.who_auto();

You probably shouldn't replace the entire prototype: User.prototype.who_auto = function () ...! — deceze
– deceze ♦, Commented Mar 3, 2021 at 12:01
Hmmm "probably"... I like your thinking... there are no definites in life... haha Thanks! — Chris Andrew
– Chris Andrew, Commented Mar 3, 2021 at 12:18

Mark Baijens · Accepted Answer · 2021-03-03 12:06:40Z

1

You overwritten the prototype instead of adding a property to the prototype. Below codes works.

class User {
  constructor(name) {
    this.name = name;
    this.chatroom = null;
  }

  who() {
    return `I am ${this.name}`;
  }
}

User.prototype.who_auto = function() {
  console.log(`Hello, I am ${this.name}`);
}

const pp = new User('peter parker');
console.log(pp);
console.log(pp.who());

pp.who_auto();

answered Mar 3, 2021 at 12:06

Mark Baijens

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2 Comments

Chris Andrew Over a year ago

``` const User = function(name) { this.name = name; this.chatroom = null; } User.prototype = { send: function(message, to) { this.chatroom.send(message, this, to); }, recieve: function(message, from) { console.log(${from.name} to ${this.name}: ${message}); } } const Chatroom = function() { let users = {}; // list of users return { register: function(user) { users[user.name] = user; user.chatroom = this; } } } ``` Mark, I've been working along with this tutorial and it is striking me as strange why the above works

Chris Andrew Over a year ago

Probably should have added that to the original post.. thanks Mark

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