Function.prototype is a function

The reason is that the ES5 spec says so:

The Function prototype object is itself a Function object (its
[[Class]] is “Function”) that, when invoked, accepts any arguments and
returns undefined.

Note it’s common in ES5 to make the prototype of some class a member of that class:

• Object.prototype is an Object object.
• Function.prototype is a Function object which returns undefined when invoked.
• Array.prototype is an empty Array object.
• String.prototype is a String object whose value is an empty String.
• Boolean.prototype is a Boolean object whose value is false.
• Number.prototype is a Number object whose value is +0.
• Date.prototype is a Date object whose [[PrimitiveValue]] is NaN.
• RegExp.prototype is a RegExp object whose data properties are like new RegExp()‘s ones.
• Error.prototype is an Error object.

I think it was standardized as such because the prototype of a class has the intrinsic properties of that class, as the instances of that class. And if it looks like a duck it should behave as a duck. So calling the methods of the prototype on the prototype itself instead of on an instance should work too.

However, ES6 didn’t like this. So it changed the behavior for those:

• Boolean.prototype is an ordinary object with no [[BooleanData]] internal slot.
• Error.prototype is an ordinary object with no [[ErrorData]] internal slot.
• Number.prototype is an ordinary object with no [[NumberData]] internal slot.
• Date.prototype is an ordinary object with no [[DateValue]] internal slot.
• String.prototype is an ordinary object with no [[StringData]] internal slot.
• RegExp.prototype is an ordinary object with no [[RegExpMatcher]] nor any of the other internal slots of RegExp instance objects.

And also for new “classes” _{^{(ES6 objects no longer have a [[Class]])}}:

• Symbol.prototype is an ordinary object with no [[SymbolData]] internal slot.
• TypedArray.prototype is an ordinary object with no [[ViewedArrayBuffer]] nor any other of the internal slots that are specific to TypedArray instance objects.
• Map.prototype is an ordinary object with no [[MapData]] internal slot.
• Set.prototype is an ordinary object with no [[SetData]] internal slot.
• WeakMap.prototype is an ordinary object with no [[WeakMapData]] internal slot.
• WeakSet.prototype is an ordinary object with no [[WeakSetData]] internal slot.
• ArrayBuffer.prototype is an ordinary object with no [[ArrayBufferData]] nor [[ArrayBufferByteLength]] internal slots.
• DataView.prototype is an ordinary object with no [[DataView]], [[ViewedArrayBuffer]], [[ByteLength]], nor [[ByteOffset]] internal slots.
• GeneratorFunction.prototype is an ordinary object with no [[ECMAScriptCode]] nor any other of the internal slots listed in Table 27 or Table 56.
• Promise.prototype is an ordinary object with no [[PromiseState]] nor any of the other internal slots of Promise instances.

However, the old behavior remains for those:

• Function.prototype is itself a built-in function object.
• Array.prototype is an Array exotic object and has the internal methods specified for such objects.

So now the reason is backwards compatibility:

The Function prototype object is specified to be a function object to
ensure compatibility with ECMAScript code that was created prior to
the ECMAScript 2015 specification.

Note this doesn’t make Function.prototype a special function. Only constructors have the prototype property:

Function instances that can be used as a constructor have a prototype
property.

There are multiple examples of non-constructor functions apart from Function.prototype, such as

• Methods in Math object:

  typeof Math.pow; // "function
  'prototype' in Math.pow; // false
  

• Some host objects:

  typeof document.createElement('object'); // "function
  'prototype' in document.createElement('object'); // false
  

• In ES6, arrow functions:

  typeof (x => x * x); // "function
  'prototype' in (x => x * x); // false