class questions

Hey, i have a few questions about classes given this code:

class Message {
  constructor(message) {
    this._message = message
  }
 
  get message() {
    return this._message
  }
}

x = new Message("example message")
console.log(x.message)

class Message {
  constructor(message) {
    this._message = message
  }
 
  get message() {
    return this._message
  }
}

x = new Message("example message")
console.log(x.message)

1) why does the this variable need to have a different identifer (in this case _) to prevent it from giving this error?

"Uncaught TypeError: Cannot set property message of #<Example> which has only a getter
    at new Example"

2) if i try to use a function that isn't a getter to return a this value, like below, it returns "ƒ getMessage() { return this._message; }" rather than "example message". Why is this? Is there anything like this with setters too?

class Message {
  constructor(message) {
    this._message = message
  }
 
  getMessage() {
    return this._message
  }
}

class Message {
  constructor(message) {
    this._message = message
  }
 
  getMessage() {
    return this._message
  }
}

3) why may you want to store a class in a variable? e.g. const x = class Message {...} Thanks in advance.

10 Replies

TOMUTO 🍅•2mo ago

for your 2nd question, it does return the msg if u call the getMessage function tho it returns a function if u dont call the method i.e u dont add ()

TOMUTO 🍅•2mo ago

doing console.log(c._message) also works btw, getter and setters r normally used for private props private props r properties that are not accessible outside the class itself so u provide with a getter and setter method for your 3rd question, when u construct a class, it basically creates an object so like {key: value} which would be the copy of the base class except with its own set of values and returns u the reference to that object, u basically store the reference to that object in the variabke otherwise how would u refer to that object

Jochem•2mo ago

For the first question: Using the getter syntax there, you defined a property called message, that doesn't store its own value but instead returns the result of running the getter whenever you try to access it. It's kind of a different way of writing it as a function, except you don't call it yourself, the code does it for you. Because it still has to store the value somewhere, you need to use a different name because message is already taken. The underscore is a pretty common convention to indicate that a property should be treated as private, and not accessed directly, so the example used an underscore to differentiate the actual stored value of the message and the getter function message It could just have been called "myMessage" or "text" or "Jeremiah". usually you'd use a getter or a getter/setter combo if you wanted to do some logic, or use some kind of caching. Say it's an expensive fetch command to actually get the message, and it's not used very often, you'd use a getter to check if _message is undefined, and then fetch the result if it's not, store it, and on subsequent uses, just return the cached value they're also used for computed values, so like... lets say you have a class that stores someone's height, and you store that height in centimeters. You may want to have a heightInInches or something, because you have a multi-region piece of software, but you wouldn't want to store a value that's dependent on a different value, because then you might forget to update one when you update the other, so you'd put some logic in to turn centimeters into inches. Very quick and dirty:

class Person {
  constructor(height) {
    this.height = height;
  }
  
  get heightInInches() {
    return this.height / 2.54;
  }
}

class Person {
  constructor(height) {
    this.height = height;
  }
  
  get heightInInches() {
    return this.height / 2.54;
  }
}

snxxwyyOP•2mo ago

Thank you both for the info, it’s super useful. So essentially you can create an instance of the class by either doing let x = new Message() and x = class Message {…}? Also with private properties, I thought that’s what static was for in classes? Or is that different?

Jochem•2mo ago

private properties are properties that are only used internally in the class, static means the class doesn't need to be instantiated to be used You don't get to call the constructor if you use x = Message. But I'm also not super familiar with defining classes in Javascript, I don't do it that often

snxxwyyOP•2mo ago

Oh okay I see, what do you mean by instantiated? Does that mean that things with static written infront of them don’t pass down to a class that extends the original?

Jochem•2mo ago

in most languages, when you write a class, you can have static and dynamic methods. Static methods can be called without having a class instance, where dynamic ones need a class instance (so an object, rather than a class) like I said though, I'm not super familiar with how JavaScript does it, because Javascript OOP is a nightmare compared to other languages I mostly just consume objects and classes from libraries

clevermissfox•2mo ago

I really struggle with classes and OOP in general myself. Just wanted to add on , a class instance is when you use it eg

const user = new User();

const user = new User();

So the instance is now user , it is the first instance and you can have as many as you want.

ἔρως•2mo ago

from your pattern, you seem to be trying to create a private variable dont do it this way: add #message = null; before the constructor, and then access to the value with this.#message its fugly, but thats how you make a private variable and these are really private, while what you have is just a cheeto lock: hopes and dreams are all that keep it "private"

snxxwyyOP•2mo ago

Ah okay I see, thank you all

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