an example of derriving keypairs with factories

derive_keypair.ts

interface CurrencyKey {
  keyIndex: number;
  privateKey: string;
  publicKey: string;
  address?: string;
  getKeyIndex(): number;
  getPrivateKey(): string;
  getPublicKey(): string;
  getAddress(): string;
}

abstract class CoinKey implements CurrencyKey {
  keyIndex: number;
  address: string;
  privateKey: string;
  publicKey: string;

  constructor(keyIndex: number) {
    this.keyIndex = keyIndex;
  }

  getKeyIndex(): number {
      return this.keyIndex
  }

  getPrivateKey(): string {
      return this.privateKey
  }

  getPublicKey(): string {
      return this.publicKey
  }

  getAddress(): string {
      return this.address
  }
}

class BitCoinKey extends CoinKey {
  constructor(masterSeed: string, keyIndex: number) {
    super(keyIndex)
    this.privateKey = `btc pri key ${masterSeed}`;
    this.publicKey = `btc pub key ${masterSeed}`;
    this.address = `btc address ${masterSeed}`;
  }
}

class EthCoinKey extends CoinKey {
  constructor(masterSeed: string, keyIndex: number) {
    super(keyIndex)
    this.privateKey = `btc pri key ${masterSeed}`;
    this.publicKey = `btc pub key ${masterSeed}`;
    this.address = `btc address ${masterSeed}`;
  }
}

interface DerivedKeyPair {
  keyIndex: number;
  publicKey: string;
  address: string;
}

interface IConstructor<T> {
  new (...args: any[]): T;

  // Or enforce default constructor
  // new (): T;
}

// instead of using an IConstructor interface, we could use less readable inline initialization for t
// function keyPairFactory<T extends CurrencyKey>(masterSeed: string, keyIndex: number, type?: { new(): T ;}) {

function keyPairFactory<T extends CurrencyKey>(masterSeed: string, keyIndex: number, type?: IConstructor<T>) {
  return new type(masterSeed, keyIndex);
}

function deriveNewKeyPair<T extends CurrencyKey>(masterSeed: string, keyIndex: number): DerivedKeyPair {
  const key = keyPairFactory<T>(masterSeed, keyIndex)

  return {
    keyIndex: key.getKeyIndex(),
    publicKey: key.getPublicKey(),
    address: key.getAddress()
  }
}

// derive keyPairs
const btcKeyPair = deriveNewKeyPair<BitCoinKey>('btcaddrts', 1)
const ethKeyPair = deriveNewKeyPair<EthCoinKey>('ethaddr', 1)

// use a factory to create a coin key
const key = keyPairFactory<BitCoinKey>('dssd', 1, BitCoinKey)