A mixin to extend multiple classes while maintaining typing.

extend-all.mixin.ts

/* eslint-disable @typescript-eslint/no-explicit-any */

/**
 * Utility type to convert a union of types into an intersection of types.
 * Ex: UnionToIntersection<{a: 1} | {b: 2}> = {a: 1, b: 2}
 */
type UnionToIntersection<U> = (U extends any ? (x: U) => void : never) extends (x: infer I) => void ? I : never;

/**
 * Generic type to represent a constructor class.
 */
type Constructor<T = {}> = abstract new (...args: any[]) => T;

/**
 * Combines multiple abstract classes into a single base class.
 * Copies prototype methods and static members from all classes.
 */
export function extendsAll<T extends Constructor[]>(
  ...bases: T
): Constructor<UnionToIntersection<InstanceType<T[number]>>> {
  abstract class Mixed {}

  for (const base of bases) {
    // Copy instance methods
    const proto = base.prototype;
    for (const key of Object.getOwnPropertyNames(proto)) {
      if (key === 'constructor') continue;
      Object.defineProperty(Mixed.prototype, key, Object.getOwnPropertyDescriptor(proto, key)!);
    }

    // Copy static references
    for (const key of Object.getOwnPropertyNames(base)) {
      if (['length', 'prototype', 'name'].includes(key)) continue;
      Object.defineProperty(Mixed, key, Object.getOwnPropertyDescriptor(base, key)!);
    }
  }

  return Mixed as any;
}