@rse/traits v1.0.3

Traits

Traits for TypeScript Classes

About

This is a small TypeScript library providing a trait (aka mixin) facility for extending classes with multiple base functionalities, although TypeScript/JavaScript technically do not allow multiple inheritance.

For this, it internally leverages the regular class extends mechanism at the JavaScript level, so it is does not have to manipulate the run-time objects at all. At the TypeScript level, it is fully type-safe and correctly derives all properties of the traits a class is derived from.

This library consists of just three API functions: trait for defining a trait (or sub-trait), the API function derive for deriving a base class from one or more defined traits, and the API type-guard function derived to ensure an object has the functionality of a trait under run-time.

Installation

$ npm install --save @rse/traits

API

The Application Programming Interface (API) of @rse/traits consists of the following parts:

• import {trait, derive, derived} from "@rse/traits" Import the three API functions.

• const Foo =trait((base) => class Foo extends base { ... } const Foo =trait([ Bar, Quux ], (base) => class Foo extends base { ... } Define a regular trait (or sub-trait) with the help of a trait factory function. A sub-trait inherits all properties of its super-traits.

• const Baz = <T extends any>() => trait((base) => class Baz extends base { ... T ... } const Baz = <T extends any>() => trait([ Bar, Quux ], (base) => class Baz extends base { ... T ... } Define a generic trait (or sub-trait) with the help of a trait factory function enclosed in a wrapping factory function for the generic type specification.

• class Sample extendsderive(Foo, Bar<baz>, Quux) { ... } Define an application class with features from a base class which is derived from one or more regular or generic traits.

• constructor () {super(...); ... } foo () { ...;super.foo(...); ... } Call the super constructor (or super method) from an application class constructor (or method).

Example

import { trait, derive } from "@rse/traits"
import { expect }        from "chai"

const Queue = trait((base) => class extends base {
    private buf: Array<number> = []
    get ()          { return this.buf.pop() }
    put (x: number) { this.buf.unshift(x) }
})
const Trace = trait((base) => class extends base {
    get () { return super.get() }
    put (x: number) { super.put(x) }
})
const Doubling = trait((base) => class extends base {
    put (x: number) { super.put(2 * x) }
})
const Incrementing = trait((base) => class extends base {
    put (x: number) { super.put(x + 1) }
})
const Filtering = trait((base) => class extends base {
    put (x: number) { if (x >= 0) super.put(x) }
})

const Queue = class Queue extends
    derive(Filtering, Doubling, Incrementing, BasicIntQueue) {}

const queue = new Queue()
expect(queue.get()).to.be.equal(undefined)
queue.put(-1)
expect(queue.get()).to.be.equal(undefined)
queue.put(1)
expect(queue.get()).to.be.equal(3)
queue.put(10)
expect(queue.get()).to.be.equal(21)

History

The @rse/traits library was developed in January 2025 by Dr. Ralf S. Engelschall. It is heavily inspired by Scala traits and the API of @ddd-ts/traits, although @rse/traits is a "from scratch" implementation for TypeScript.

License

Copyright © 2025 Dr. Ralf S. Engelschall (http://engelschall.com/)

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.