How to Extend Functions with ES6 Classes and Access Instance Variables?

Extending Functions with ES6 Classes

ES6 introduces the ability to extend special objects like the Function object. This allows us to create classes that inherit from functions and behave like functions when called. However, implementing the logic for such calls requires careful consideration.

Understanding the Function Prototype

When calling a class as a function, the this keyword refers to the global object (window in a browser environment) rather than the class instance. To access the class instance, we cannot use the traditional super() call, as it expects a code string for the Function constructor.

Hardcoding the Value

One approach is to hardcode the value into the constructor:

class Smth extends Function {
  constructor(x) {
    super("return " + JSON.stringify(x) + ";");
  }
}

(new Smth(256))() // Returns 256

However, this is not ideal as it relies on hardcoded values.

Leveraging Closures

To enable a closure that can access instance variables, we can create a nested function within the constructor:

class Smth extends Function {
  constructor(x) {
    // Refer to `smth` instead of `this`
    function smth() {
      return x;
    }
    Object.setPrototypeOf(smth, Smth.prototype);
    return smth;
  }
}

(new Smth(256))() // Returns 256

Abstraction with ExtensibleFunction

To abstract this pattern, we can create an ExtensibleFunction class:

class ExtensibleFunction extends Function {
  constructor(f) {
    return Object.setPrototypeOf(f, new.target.prototype);
  }
}

class Smth extends ExtensibleFunction {
  constructor(x) {
    super(function() {
      return x;
    });
  }
}

(new Smth(256))() // Returns 256

This approach provides a reusable way to create extensible functions with access to instance variables.

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