CODEWITHSUNDEEP
javascriptecmascript-6getter
Achshar
Achshar

Reputation: 5243

Lazy getter doesn't work in classes

class a {
    get b() {
        delete this.b;
        return this.b = 1;
    }
}

var c = {
    get b() {
        delete this.b;
        return this.b = 1;
    }
}

console.log(c.b); // works as expected
console.log((new a()).b); // throws error

The above code should work fine but the last line throws.

Uncaught TypeError: Cannot set property b of # which has only a getter(…)

Clearly the getter is not being deleted in class whereas it works fine in object. I am on latest stable chrome.

Lazy Getter MDN Entry

Upvotes: 11

Views: 4737

Answers (3)

Bergi
Bergi

Reputation: 665256

The getter of the class sits on the .prototype object, not on this, that's why your attempt to delete it fails (and, as Jeremy points out, it is not deletable).

You can however simply create an own property on the instance that shadows the getter:

class a {
    get b() {
        Object.defineProperty(this, "b", { value: 1, writable: false, configurable: true })
        return this.b;
    }
}

var c = new a;
console.log(c.b); // 1

We have to use Object.defineProperty() as a simple assignment would find the inherited property that has no setter and throws.

Upvotes: 21

Redu
Redu

Reputation: 26191

Well obviously this is about constructors. When you neglect the constructor the property you define with the getter gets created at the constructors prototype hence the correct way of doing the job probably should be like as follows;

class a {
    get b() {
        delete this.constructor.prototype.b;
        return this.constructor.prototype.b = 1;
    }
}
z = new a();
z.b; // 1

Upvotes: 0

Jeremy J Starcher
Jeremy J Starcher

Reputation: 23863

By design, class properties are not deletable -- reference: https://github.com/jeffmo/es-class-fields-and-static-properties

Instance Field Initialization Process

The process for executing a field initializer happens at class instantiation time. The following describes the process for initializing each class field initializer (intended to run once for each field in the order they are declared):

  • Let instance be the object being instantiated.
  • Let fieldName be the name for the current field (as stored in the slot on the constructor function).
  • Let fieldInitializerExpression be the thunked initializer expression for the current field (as stored in the slot on the constructor function).
  • Let initializerResult be the result of evaluating fieldInitializerExpression with this equal to instance.
  • Let propertyDescriptor be PropertyDescriptor{[[Value]]: initializerResult, [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: false}. Call instance.[[DefineOwnProperty]](fieldName, propertyDescriptor).

Upvotes: 1

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