Creating Callable Objects in JavaScript

A callable object is a data structure that behaves as both an object and a function.

You can

• access and assign properties obj.bar,
• call methods obj.foo(), but also
• call the object directly obj(), as if it were a function.

The direct call is like calling a method of obj which has access to the object’s properties through its this context.

Callable Objects can also be thought of as stateful functions. Functions are inherently single instance, stateless procedures. Callable Objects are instantiated, stateful procedures.

See package callable-object for a simple implementation of a callable object in JavaScript. See also the example at callable-instance.js.

Goal

We want to create a Class/constructor that we can use to create callable objects that redirect their calls to a method named _call.

We want this redirect so that we can inherit from our Class and easily override and extend the _call method with new functionality, without having to worry about the inner workings of the callable object.

To do this, we’re going to need to inherit from the Function constructor, which inherits from Object, and allows us to create both an object and a dynamic function.

Our main hurdle is giving a function object a reference to itself.

In order to have a reference to the _call method, the function part of our function object, generated by our Callable class/constructor, must have a reference to itself.

The Solutions

We want to create an extensible Callable class that maintains proper and correct inheritance in JavaScript, and allows us to call the objects it constructs as functions, with a reference to themselves, redirecting those calls to an overridable method _call.

References

1. Creating Callable Objects in JavaScript by Adrien. 2019.
2. gist
3. repl.it