CODEWITHSUNDEEP
javascriptclass
Bagel03
Bagel03

Reputation: 715

Modify the Constructor Function of a Class

I already have a function for modifing other class methods:

class MyClass {
    constructor() {
        this.num = 1;
    }
    inc(){
        this.num++;
    }
}

runAfterMethod(classHandle, methodName, executeAfter) {
    const oldHandle = classHandle.prototype[methodName];
    classHandle.prototype[methodName] = function () {
        const returnValue = oldHandle.apply(this, arguments);
        //@ts-ignore
        executeAfter.apply(this, arguments);
        return returnValue;
    };
}

runAfterMethod(MyClass, "inc", function() {console.log(this.num)})
new MyClass().inc(); // Prints 2

However, this does not work for the constructor of the class (because it is technically the class itself)

What I would really be happy with is something that mimics this behavior:

class MyClass {
    constructor() {
        this.num = 1;
    }
}

extendConstructor(MyClass, function(){
    this.str = "hello world"
})

new MyClass(); // num: 1, str: "hello world"

I have looked at things like How to modify the constructor of an ES6 class however, they all require = somewhere, which does not work inside of a function (where that only changes the value of the class inside the function, not the class itself)

Upvotes: 1

Views: 605

Answers (1)

CertainPerformance
CertainPerformance

Reputation: 370729

Mutating the existing class is really weird. What would be much less of a code smell IMO would be to create a wrapper around the class that instantiates the class, then calls it with your custom callback, and then returns the instance - it'd be a lot more straightforward. Would that work for your purposes?

class MyClass {
    constructor() {
        this.num = 1;
    }
}

const extendConstructor = (theClass, callback) => function(...args) {
  const instance = new theClass(...args);
  callback.call(instance);
  return instance;
};
const Extended = extendConstructor(MyClass, function(){
    this.str = "hello world"
})

console.log(new Extended()); // num: 1, str: "hello world"

Upvotes: 1

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