ThaUnknown / async-inheritance

Provide async constructor functionality to JavaScript classes

async-inheritance

This module provides a class and the symbols necessary to create classes whose constructors return promises, while staying as close to JavaScript's normal class inheritance functionality as possible.

Usage

The simplest async class looks like this:

const { Async, constructor, Super } = require("async-inheritance")

class MyClass extends Async {
    async [constructor]() {
        await this[Super]()
        // async code
    }
}

(async () => {
    const instance = await new MyClass()
    console.log(instance instanceof MyClass) // true
})()

If you can't extend a function, simply wrap it:

const { wrap } = require("async-inheritance")
const { EventEmitter } = require("events")

const WrappedClass = wrap(class extends EventEmitter {
    async [constructor]() {
        await asyncCode()
    }
})

Classes that extend Async can then be extended further:

const { Async, constructor, Super } = require("async-inheritance")
const timers = require("timers/promises")

class MyClass1 extends Async {
    async [constructor](construction_delay) {
        await this[Super]()
        await timers.setTimeout(construction_delay)
    }
}

class MyClass2 extends MyClass1 {
    async [constructor]() {
        await this[Super](100)
        console.log("Finished constructor")
    }
}

(async () => {
    await new MyClass2()
})()

If a child class overrides [[constructor]] it must call [[Super]], but the constructor can be omitted entirely with no issue:

const { Async, constructor, Super } = require("async-inheritance")
const timers = require("timers/promises")

class MyClass1 extends Async {
    async [constructor](construction_delay) {
        await this[Super]()
        await timers.setTimeout(construction_delay)
    }
    do_something() {
        // do something
    }
}

class MyClass2 extends MyClass1 {
    do_something() {
        super.do_something()
        // do something more
    }
}

class MyClass3 extends MyClass2 {
    async [constructor]() {
        await this[Super](100)
        console.log("Finished constructor")
    }
}

(async () => {
    await new MyClass3()
})()

Just like with normal constructors, the async constructor can return an object:

const { Async, constructor, Super } = require("async-inheritance")

class MyClass extends Async {
    async [constructor]() {
        await this[Super]()
        return {my_property: 1}
    }
}

(async () => {
    const instance = await new MyClass()
    console.log(instance.my_property === 1) // true
})()

Notes

• Overriding the normal constructor will break the async constructor's functionality
• Acessing this before calling await this[Super]() may cause unexpected behavior (it does NOT throw an error the way that normal JavaScript class inheritance does)

Compatibility

This module requires the following features:

• Symbol
• Promise
• classes
• async/await
• const
• rest/spread operator
• generator functions
• Object.getPrototypeOf
• Object.getOwnPropertyNames
• Object.getOwnPropertySymbols