javascript - Run a function every time any function in class is called using JS

Question

I have simple class like this:

module.exports = class MyClass {

    function middleware() {
        console.log('call me before')
    }

    function a() {

    }
    function b() {

    }

    function c() {

    }
}

So Idea is, when someone call function a, b, c I want call middleware before execute a, b, c. How can I do it?

So, I can put middleware() to each function, but I want some dynamic way how to do this.

Answer 1

You could rewrite all the methods of the classes prototype by iterating over all own property names (Object.keys or for..in would not work here as class methods are not enumerable) and then replacing the original methods by a new method that calls the original method but also calls the middleware. Through that the classes behaviour doesnt change, but the middleware gets called.

 class MyClass {
    a() { console.log("a"); }
 }

 function middleware() { 
    console.log("works");
 }

 for(const key of Object.getOwnPropertyNames(MyClass.prototype)) {
     const old = MyClass.prototype[key];
     MyClass.prototype[key] = function(...args) {
       middleware(...args);
       old.call(this, ...args);
     };
 }

 (new MyClass).a();