How to Access Class Instance from Function Invocation when Extending Functions with ES6 Classes

Extending Functions with ES6 Classes

In ES6, programmers have the ability to extend special objects, including functions. By employing inheritance, it becomes feasible to derive classes from functions. While these extended objects can be invoked like functions, implementing the appropriate logic for these calls can pose a challenge.

One significant question arises in this scenario: how to obtain a reference to the class instance when the object is invoked as a function, considering that regular methods gain access via this? Unfortunately, this reference points to the global object (window) in these cases.

Solution:

To tackle this issue, one can employ the concept of closures, thereby creating a returned function that encapsulates access to instance variables. Here's an illustration:

class Smth extends Function {
  constructor(x) {
    super(() => { return x; });
  }
}

In this example, the super expression within the constructor initiates the function constructor, requiring a string representing the code to be executed. However, accessing instance data is not straightforward, so a hardcoded approach is adopted, yielding the desired result:

console.log((new Smth(256))()); // Logs: 256

An alternative approach to achieve the same objective involves manipulating the prototype chain:

class Smth extends Function {
  constructor(x) {
    const smth = function() { return x; };
    Object.setPrototypeOf(smth, Smth.prototype);
    return smth;
  }
}

This method offers greater flexibility by allowing the returned function to be a closure capable of accessing instance variables.

Furthermore, it's possible to abstract this functionality into a reusable utility:

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

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

This approach creates an extra level of indirection in the inheritance hierarchy, but it can be beneficial in certain scenarios. Additionally, one can avoid this by utilizing the following construct:

function ExtensibleFunction(f) {
  return Object.setPrototypeOf(f, new.target.prototype);
}
ExtensibleFunction.prototype = Function.prototype;

Please note, however, that Smth will not dynamically inherit static Function properties in this case.

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