How do you create a javascript class with immutable properties

Question

This question is specifically about preventing unwanted properties from being added to a javascript "class". I have a class called Animal.

function Animal(){
this.name="";
this.type="";
//20 other properties
}

What would be the easiest way for a user to create their own Animal and add 20 properties. I want to also prevent the user from accidentally adding incorrect properties.

my current method:

    var myAnimal= new Animal();
    myAnimal.name="fluffy";
    myAnimal.type="cat";
    myAnimal.typo="cat";

//adding 20 more properties would require typing myAnimal 20 more times Plus if a user makes a typo it would add it as a new property.

I was hoping there would be something like this:

myAnimal=new Animal{
name:"fluffy",
type:"cat";
typo:"cat" //would throw an error
}

I've looked into Object.freeze Object.seal, Object.preventExtensions but not sure how they apply to classes.

Answer 1

You can Object.seal(this) at the end of the constructor to prevent new properties from being added:

function Animal() {
    this.name = "";
    this.type = "";
    // 20 other properties
    Object.seal(this);
}

and you can take an object with initial values in the constructor:

function Animal(initial) {
    this.name = "";
    this.type = "";
    // 20 other properties
    Object.seal(this);
    Object.assign(this, initial);
}

used like so:

myAnimal = new Animal({
    name: "fluffy",
    type: "cat",
    typo: "cat",
});

with the downside that the error doesn’t point directly at typo like it would if you used multiple assignment statements.