Intercept calls to constructor

Question

I am having a bit of trouble intercepting constructor calls to a library (so I can replay them later) while still maintaining the prototype chain. More concretely, I am working with a library (ThreeJS, but could be any library), and some code that uses this library. What I want to do is write a piece of code that modifies the library objects, so I can run a block of code every time a constructor is called.

Example: when a new scene is created, I want to print "new Scene created" to the console.

var scene = new THREE.Scene();

When the constructor takes arguments, I also want to log these arguments.

Answer 1

I'm not sure about this, but... you could try something like...

// Backup the original constructor somewhere
THREE._Scene = THREE.Scene;

// Override with your own, then call the original
THREE.Scene = function() {
  // Do whatever you want to do here..
  THREE._Scene.apply(this, arguments);
}

// Extend the original class
THREE.Scene.prototype = Object.create(THREE._Scene.prototype);

Answer 2

Starting from @bvaughn code, this worked for me:

THREE._Scene = THREE.Scene;

THREE.Scene = function() {
  const Scene = Function.prototype.bind.apply(THREE._Scene, arguments);
  const scene = new Scene()
  console.log("Intercepted scene:", scene)
  return scene;
}

THREE.Scene.prototype = Object.create(THREE._Scene.prototype);

// Test
const scene = new THREE.Scene()
// Prints "Intercepted scene:", ....