ooplus.js

Utilities to improve object-oriented programming in JavaScript and Typescript.

Getting Started

You can get ooplus.js on npm.

Use Cases

Add functionalities an existing class

import { enhance, property } from "ooplus-js"

class MyClass {
    baseValue = 40;
}

interface EnhancedClass extends MyClass {
    newValue: number;
    newProperty: number;
    newMethod();
}

enhance(MyClass, {
    newValue: 2,
    newProperty: property({
        writable: true
    }),
    newMethod: function(this: EnhancedClass) {
        this.newProperty = this.newValue + this.baseValue;
    }
});

This code enhances MyClass to add two properties newValue, newProperty and a method newMethod.

A property can be added in two ways: with the property function and a PropertyDescriptor, or a direct value. Method is actually a property too, with a type of function.

Calling super

import { enhance, $super } from "ooplus-js"

class BaseClass {
    protected /*virtual*/ action() { 
        console.info("base action")
    }
}

class DerivedClass {
    
}

interface EnhancedDerivedClass extends DerivedClass {
    action();
}

enhance(DerivedClass, {
    action(this: EnhancedDerivedClass) {
        $super(this).action();
        console.info("derived action")
    }
})

The $super function here mimics the ES6 super syntax, to call a method defined in the base/ancestor class.

Dynamic inheritance

class BaseClass {
    constructor(readonly value: number) {}
}

interface DerivedClass extends BaseClass {
    value1: number;
    value2: number;
}

const derivedClass = extend(BaseClass, {
    preConstructor(value1: number, value2: number) {
        return [ value1 + value2 ];
    },
    
    constructor(value1: number, value2: number) {
        this.value1 = value1;
        this.value2 = value2;
    }
});

const instance = new derivedClass(22, 20);

The extend function works like the enhance function, but rather than enhancing the existed class, it creates a new class inheriting from the specified base class. This example dynamically creates a class derivedClass.

While using the enhance function, the new class' constructor is split into two functions: a preConstructor and a constructor. Both functions will receive the same parameters. The pre-constructor is like code before the super call which calls the base class' constructor in a typical ES6 class constructor code, it can handle pre-processing of the arguments, and should return an array of parameters which will be used to call the base class' constructor. The constructor function is like the code after the super call, which can do all sorts of initializations. The code above is analog to the following ES6 code:

class BaseClass {
    constructor(readonly value: number) {}
}

class DerivedClass extends BaseClass {
    constructor(value1: number, value2: number) {
        super(value1 + value2);
        this.value1 = value1;
        this.value2 = value2;
    }
});

Note in the preConstructor, the this object in the preConstructor is called a courier object, anything stored in it will be assigned to the instance created later. However it does not represent the class instance to be created. which will not be available until the constructor function (just like you can't use this before the super call in an ES6 class constructor).