How to Monkey Patch Js Class Constructor

Question

I am fairly new to js and I have a problem that seems I can solve by modifying the constructor of the base class. This patch here is not applied

// original class in lib
class BasePrinter {
  constructor() {
    console.log("BasePrinter constructor");
  }
}

// patching
function patchBasePrinter() {
  const originalConstructor = BasePrinter.prototype.constructor;

  BasePrinter.prototype.constructor = function(...args) {
    console.log("Patched constructor logic before");

    originalConstructor.apply(this, args);

    console.log("Patched constructor logic after");
  };
}

// Apply the patch
patchBasePrinter();

const printer = new BasePrinter(); 
// Output: BasePrinter constructor

// Expected Output:
// Patched constructor logic before
// BasePrinter constructor
// Patched constructor logic after

The other posts ive seen on this are either inconclusive or end with a solution of patching a method instead.

The real use case is to apply new logic when any class property is changed. The system is already reactive. I just want to add new logic and all the techniques I can find have to do with patching the constructor class

"This does not work" - why not? What happens instead? What's the expected behaviour? — Nico Haase
– Nico Haase, Commented Jul 31, 2024 at 8:50
I have edited the code for clarity. Basically the patch is not applied — Tony
– Tony, Commented Jul 31, 2024 at 9:08
does the patched constructor need to run before the original constructor? If not, I may have a solution based on this similar question — Jaromanda X
– Jaromanda X, Commented Jul 31, 2024 at 9:33

Jaromanda X · Accepted Answer · 2024-07-31 09:58:12Z

2

Based on this similar question, with a little extra work, this could do what you need.

For example:

class BasePrinter {
    constructor() {
        console.log("BasePrinter constructor");
    }
}
class X extends BasePrinter {
    constructor(...args) {
        console.log("Before original constructor - usual limitations apply for extended class");
        super(...args);
        console.log("Your logic here");
    }
}

X.prototype = BasePrinter.prototype;
X.prototype.constructor = X;
BasePrinter = X;
new BasePrinter

answered Jul 31, 2024 at 9:58

Jaromanda X

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