Why can't I set the 'prototype' of a function created using 'bind'?

As I understood, every function has a prototype object.

Well, there are exceptions to every rule :-) You found one: bound functions don't have a .prototype property because they don't need it. When you call a bound function with new, it calls the original function as a constructor, using the original's .prototype object as the prototype of the new instance.

In fact, since ECMAScript 6 many functions don't have a .prototype property with an object, because they are not constructors - they cannot be called with new so they don't need it. Among those are

• arrow functions (() => {…})
• methods (method() { … } in object literals and classes)
• builtin non-constructor functions (like Math.sin)

See the specification:

Function.prototype.bind ( thisArg , ...args)

[...]

NOTE 1 Function objects created using Function.prototype.bind are exotic objects. They also do not have a prototype property.

The returned function from .bind() has no prototype object. You can give it one:

bar.prototype = { newprop: "new" };

It is not true that "every function has a prototype object". Every function can have a prototype object, but the value of the "prototype" property can be anything, including null or undefined.

Additionally, there are "special" functions that may not behave like ordinary functions in all cases.