Can’t move a method that uses super

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I'm reading this book. Check section "15.6.4.2 Pitfall: A method that uses super can’t be moved"

It states the following

You can’t move a method that uses super: Such a method has the internal slot [[HomeObject]] that ties it to the object it was created in. If you move it via an assignment, it will continue to refer to the superproperties of the original object.

So, I tried to call it with a different object, also assign instance method to a different object but it seems not to work. It refers to new object's properties. Probably, I didn't get correctly what the author means. So, could anyone, please, provide an example?

Hers is a small demo and the code below

class A {   get a() {     return 1;   }    sayHi() {     console.log(this.a)   } }  class B extends A {   sayHi() {     super.sayHi();   }; } var obj = {   a: 4 }; let b = new B(); b.sayHi(); // logs 1  b.sayHi.call(obj); // logs 4  obj.sayHi = b.sayHi; obj.sayHi(); // logs 4 

 


Your code is OK, because this works when you move a method, but super doesn’t (and your code only really tests this). The following code tests super:

class Bird {   getName() {     return 'Bird';   } } class Mammal {   getName() {     return 'Mammal';   } } class Human extends Mammal {   getName() {     return super.getName();   } } class Duck extends Bird { } // Moving method .getName() from Human.prototype to Duck.prototype Duck.prototype.getName = Human.prototype.getName;  console.log(new Duck().getName()); // 'Mammal' (not 'Bird') 

To understand the result, you need to understand how super works – it uses the internal property [[HomeObject]] that is stored in the method itself, it does not rely on this. That is, Human.prototype.getName() internally works as follows:

Human.prototype.getName = Object.assign(   function me() {     return me.__HomeObject__.__proto__.getName();   },   { __HomeObject__: Human.prototype } ); 

More details are explained in the book: http://exploringjs.com/es6/ch_classes.html#superproperties

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