From the MDN docs for the *standard* [`setPrototypeOf` function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/setPrototypeOf)  as well as the non-standard [ `__proto__` property](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/proto):
&gt; Mutating the [[Prototype]] of an object, no matter how this is accomplished, is strongly discouraged, because it is very slow and unavoidably slows down subsequent execution in modern JavaScript implementations.



Using `Function.prototype` to add properties is *the* way to add member functions to javascript classes. Then as the following shows: 

    function Foo(){}
    function bar(){}
    
    var foo = new Foo();
    
    // This is bad: 
    //foo.__proto__.bar = bar;
    
    // But this is okay
    Foo.prototype.bar = bar;
    
    // Both cause this to be true: 
    console.log(foo.__proto__.bar == bar); // true

Why is `foo.__proto__.bar = bar;` bad? If its bad isn&#39;t `Foo.prototype.bar = bar;` just as bad? 

Then why this warning: ***it is very slow and unavoidably slows down subsequent execution in modern JavaScript implementations***. Surely `Foo.prototype.bar = bar;` is not that bad.


**Update** Perhaps by mutation they meant reassignment. See accepted answer. 

All Prototype Chain Solutions on Gang of Coders

Total of 1 Prototype Chain Solutions

Why is mutating the [[prototype]] of an object bad for performance?