Javascript - Reassignment of class instance from its own method

Question

is it possible to clone idiomatically a javascript class instance from within of its own method? Others languages like Java allow me to overwrite/clone the current instance by either construct overload or this overwrite. Javascript does not seems to like it, and throws the error SyntaxError: Invalid left-hand side in assignment. THere is an example below for this verbose explanation.

class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }

  renew() {
    this = new Car('Ferrari', 2021);
    return this
  }
}

car = new Car('Ford', 206)
car.renew()

You seem to be confused, you definitely can't do this in Java — Nick Bailey
– Nick Bailey, Commented Mar 19, 2022 at 21:47
"other languages like Java allow me to overwrite/clone the current instance by either construct overload or this overwrite." - no, they don't either — Bergi
– Bergi, Commented Mar 19, 2022 at 21:47

CertainPerformance · Accepted Answer · 2022-03-19 21:43:13Z

1

this can't be assigned to. For what you want, you must either:

Return a new instance from the method, and reassign it outside the method

class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }

  renew() {
    return new Car('Ferrari', 2021);
  }
}

let car = new Car('Ford', 206)
car = car.renew()
console.log(car.year);

Or mutate the instance inside the method

class Car {
  constructor(name, year) {
    this.name = name;
    this.year = year;
  }

  renew() {
    Object.assign(this, new Car('Ferrari', 2021));
  }
}

let car = new Car('Ford', 206)
car.renew()
console.log(car.year);

answered Mar 19, 2022 at 21:43

CertainPerformance

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2 Comments

Bergi Over a year ago

Maybe even this.constructor('Ferrari', 2021) - oh, wait, that works only in ES5 but not with class :-/

Bruno Peixoto Over a year ago

This is correct, but not this. :-(

Nick Bailey · Accepted Answer · 2022-03-19 21:46:10Z

0

No, it is not possible to do this. this is immutable. Your options would either be to modify the properties of the class, or create a new class instance and assign it to a your variable in enclosing scope.

I'm also not sure what language you are using that allows assignment to a this reference. It's not possible in Java, C#, Scala, or Ruby. It's technically possible in Python, but doesn't have the semantics you describe (e.g. it does not mutate the object). You can't even do it in C++.

answered Mar 19, 2022 at 21:46

Nick Bailey

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