Cannot assign to read only property 'name' of object '[object Object]'

Question

The following code will throw an error only for the name property. It could be fixed by specifying name property as writable in Object.create

Answer 1

You cannot modify the name property of a function. The descriptor says it is not writable...

var BaseClass = function (data) {
  Object.assign(this, data);
};

console.log(Object.getOwnPropertyDescriptor(BaseClass, 'name'));

But since it is configurable, you could use Object.defineProperty().

var BaseClass = function (data) {
  Object.assign(this, data);
};

Object.defineProperty(BaseClass, 'name', {
  writable: true,
  value: 'Foo'
});

console.log(BaseClass.name);

---

EDIT

I'm back! So... As I said previously in comments, I think I have identified your problem. I answered a bit too fast and did not see that your ES5 inheritance is wrong.

ExtendedClass.prototype = Object.create(BaseClass); is not what you want to do. Doing so means the prototype of ExtendedClass becomes a constructor function. This obviously generates an unexpected behavior.

function BaseClass(data) {
  console.log(this instanceof BaseClass); // "this" is not an instance of "BaseClass"
  console.log(this instanceof Function); // "this" is a function
  console.log(this.name); // "this" is "BaseClass"
  
  Object.assign(this, data);
}

function ExtendedClass() {
  BaseClass.apply(this, arguments);
}
ExtendedClass.prototype = Object.create(BaseClass);

new ExtendedClass({ type: 'foo' });

In your code, this is a function and refers to BaseClass. That is why you are not allowed to modify its name...

In fact, when working with inheritance in JavaScript, you generally need these two lines:

ExtendedClass.prototype = Object.create(BaseClass.prototype);
ExtendedClass.prototype.constructor = ExtendedClass;

Here is a valid implementation:

function BaseClass(data) {
  console.log(this instanceof BaseClass); // "this" is an instance of "BaseClass"
  console.log(this instanceof Function); // "this" is not a function
  console.log(this.name); // "this" has no name yet
  
  Object.assign(this, data);
}

function ExtendedClass() {
  BaseClass.apply(this, arguments);
}
ExtendedClass.prototype = Object.create(BaseClass.prototype);
ExtendedClass.prototype.constructor = ExtendedClass;

var instance = new ExtendedClass({ name: 'foo' });

console.log(instance.name); // foo
console.log(BaseClass.name); // BaseClass
console.log(ExtendedClass.name); // ExtendedClass

Answer 2

Used ES7+ or TypeScript spread operator feature to overcome this

obj = { ...obj, name: { first: 'hey', last: 'there'} }

Answer 3

If you get this error in Angular+Typescript+NgRX:

You can use the spread operator to take a shallow copy of a readonly object to make it readable, however you may not want this depending on your situation.

let x = [...y];

If you're using Redux / NgRX, there's a chance your selector could be returning a readonly object with a reference to the store, which can throw exceptions when trying to alter that object property via template binding. Depending on your situation, you can take a deep copy to remove the store reference.

let x = JSON.parse(JSON.stringify(y));

Answer 4

The name is reserved property of Function object to which you are trying to set it in. You cannot set it.

documentation for name property is at MDN.

Answer 5

If you get this error in Angular+TypeScript:

WRONG / INVALID:

@Output whatever_var = new EventEmitter();

GOOD / CORRECT:

@Output() whatever_var = new EventEmitter();

Answer 6

I ran into this issue in Angular, while setting a local variable from ActivatedRoute's queryParams, and attempting to conditionally either override or merge... Duplicating beforehand did the trick:

updateQp(qp = {}, force = false) { 
    let qpLoc = Object.assign({}, this.queryParamsLocal)
    this.queryParamsLocal = force ? qp : Object.assign(qpLoc, qp)
}