Understanding function objects in JavaScript

Question

I am new to JavaScript and I am following Douglas Crockford's book, The Good Parts.

It says:

Every function object is also created with a prototype property. Its value is an object with a constructor property whose value is the function. This is distinct from the hidden link to Function.prototype.

I understand that function objects are linked to Function.prototype, but what does the above line mean?

Can someone simplify it for me?

Answer 1

Every function object is also created with a prototype property.

var fn = function() { };
fn.hasOwnProperty("prototype"); // true

That is, for every function in JavaScript, each one has a prototype property, just like any other JavaScript object.

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Its value is an object with a constructor property whose value is the function.

The object that prototype points to has a constructor property which points to the original function.

fn.prototype.constructor === fn // true;

That is, you can derive the constructor function from a constructed object by looking at obj.prototype.constructor (unless it's been overwritten).

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This is distinct from the hidden link to Function.prototype

The function you create's prototype object is not the same as the Function's prototype.

fn.prototype != Function.prototype // true

That is, if you augment Function.prototype, then the methods will be available on function references. If you augment your function's prototype, then those methods will be available on constructed objects using that constructor.

Answer 2

It means that a function is an instance of a class named Function.

An instance is a reproduction of a model, a class, that conveys methods and attributes.